China is North Korea's main ally and also the impoverished Communist country's largest trading partner.
The South Korean institute said in January the North imported 2,174 tons of Chinese grain, or only 9.2 percent of the imports in December, and 2 tons of Chinese fertilizer, or only 20 percent of December imports.
"The steep decline in the North's grain imports from China is very unusual, even considering the past trend of grain imports decreasing every January," said Kwon Tae-jin, a researcher at the institute, Yonhap News reported.
"The biggest reason is because China has recently begun to restrain grain exports in order to meet growing domestic demand. Bilateral tension caused by the North's rocket launch and nuclear test over the past months may also have affected the two-way grain trade."
North Korea has been under tough U.N. Security Council sanctions ever since its two nuclear tests in 2006 and 2009. The sanctions were tightened after the North's long-range rocket launch in December and its third nuclear test last month. China joined the rest of the international community in condemning the nuclear test.