China for years nudged Kim Jong-il to embrace economic reforms, but now he is dead will fear that any changes will come too quickly and unpredictably, threatening Beijing's hold on its needy yet distrustful neighbour.
In China's opaque relations with North Korea, much can rest on a single phrase, and one word that Beijing used in its formal reaction to the death of Kim -- distressed, which can also be translated as shocked -- conveyed some of the surprise and uncertainty probably weighing on Chinese policy-makers.
This is really going to throw a wrench in Chinese plans, said Stephanie Kleine-Ahlbrandt, the North East Asia project director with the International Crisis Group.
They, frankly, felt that Kim Jong-il was going to be around for a couple of years longer. They'll be a lot more nervous about what happens next.
From China's perspective, much now depends on Kim's twenty-something son, Kim Jong-un, and whether he can secure his succession quickly.
Protecting stability on China's 1,415-km (880-mile) frontier with the North and throughout the region will be paramount, particularly with Beijing's leaders grappling with their own attention-consuming succession from late next year, when President Hu Jintao steps down as Communist Party chief.
This has really come out of the blue. It's not like it had been rumoured for a while giving everyone time to properly prepare, said Cai Jian, an expert on Korean affairs at Fudan University in Shanghai.
China's biggest worry will be over North Korea's stability, and China's aim will be to ensure the country remains stable, said Cai. I think security will be stepped up in North Korea, and China is also likely to tighten security along the border.
ECONOMIC CHANGE SEEN COMING
After a decent interval, North Korea will probably pursue economic changes that will present opportunities and worries to Beijing, said several Chinese experts.
In economic policy, I think there will be major changes now that Kim Jong-il has passed away, and also in economic opening up there'll also be significant steps, said Wei Zhijiang, director of the Korea studies institute at Zhongshan University in south China.
The generation of Kim Jong-un have a strong awareness that North Korea can't remain isolated from the world.
Beijing will hope the younger Kim will embrace more measured economic reforms that will ease the chronic shortages enduring by North Koreans.
But any wider opening by the North to South Korea and its allies could dilute Chinese economic and political influence, something that fuels strategic worries in Beijing. And China could also worry that any changes could spiral beyond the grip of Pyongyang's leaders and their Chinese mentors.
In the past 18 months, Kim, who previously rarely travelled abroad, visited China four times. During Kim's visit in May the two sides vowed that their alliance, sealed in blood, would pass on to their successors.
Kim's successor, Kim Jong-un, is untested and largely unknown to the outside world.
The Kim Jong-il era is over, and the post-Kim Jong-il era has begun, and this era will certainly bring change to the Korean peninsula. That's without any doubt, said Zhu Feng, a professor of international relations at Peking University who specialises in East Asian security issues.
The issue of primary concern now is not whether North Korea will maintain political stability, but what will be the nature of the new political leadership, and what policies will it pursue at home and abroad, said Zhu.
In the short-term, there won't be new policies, only a stressing of policy stability and continuity. So soon after Kim Jong-il has died, no leader will dare say that an alternative policy course is needed.
China sees North Korea as a strategic barrier against the United States and its regional allies. But that barrier comes with an economic and diplomatic price.
China's trade and aid are crucial to Pyongyang's survival, but bring only puny economic gains to Beijing.
In October 2006, North Korea held its first nuclear test explosion, defying public pleas from China, and nuclear disarmament negotiations hosted by Beijing have languished for years without fresh progress or even fresh talks.
I think it's ultimately good news, Zhang Liangui, an expert on North Korea at the Central Party School in Beijing, said of Kim's death.
I think it's good news, because North Korea will finally have to change. Whether those changes will be for the better or the worse, we'll have to wait and see. But there's no doubt that change is needed and inevitable.
(Editing by Brian Rhoads and Alex Richardson)