There is a growing debate on the impacts of cultivated land change on China's grain security. This paper examines the changes of the area of cultivated land and its potential agricultural productivity in China using satellite images. We find that between 1986 and 2000, China recorded a net increase of cultivated land, which almost offset the decrease in average potential productivity, or bioproductivity. Therefore, we conclude that conversion of cultivated land did not threaten China's national food security. Although there was decline in cultivated land due to "Green for Grain Program" in recent years, it had little impacts on China's grain security. China needs to develop rational long-run land use plans. Stopping conversion will hurt economic growth.