The conversion from farmland to forest or grassland is a great action of eco-environmental construction in Northwest China, but necessarily it may directly influence the grain production of that area.Proceeding from the conversion scale and the grain response in Northwest China, and taking county as the unit, this study discusses the question of cultivated land and grain production in the five provinces ( autonomous regions) in Northwest China. According to analysis: excluding other factors,if all the farmlands with a slope being greater than 15 degrees are to be converted, the defarming area will reach 392.8 10⁴ ha, or 23.5% of the total farmland area.Considering the present grain-production and consumption level, the authors think it is impossible to convert farmland with a slope greater than 15 degrees; if all the farmlands having a slope greater than 25 degrees are converted,according to the legal provisions,the defarming area will reach 153.6 10⁴ ha, or 9.2% of the total farmland area, and the output reduction of grain will be 71.21×10⁴ t ,or about 2.2% of the region's output in 1996. The impact on per capita grain available of the whole region is slight.On the premise of per capita 0.1 ha of cultivated land, if farmlands with a slope greater than 15 degrees are converted, then the defarming area will be 347.80ha,20% of the present land area, and the corresponding deduction of grain is going to be 234.5 10⁴ t, which is 7.3% of the 1996 grain production.The percentage of grain deduction is about 10 in Gansu, Shaanxi and Qinghai provinces.The impact of conversion on output of grain is obvious. In view of the conversion scale and its influence on grain production, this research proposes two policy advises: (1) To coordinate conversion from farmland to forest or grassland with local ecological environmental construction, make overall plans, formulate respectively guidance for different types of areas, give top priority to what is the most important, implement by stages and in groups and carry out with the slope transformation of cultivated land in step.(2)To implement "storing food in land" project, develop reserve land resources in a planned way, strengthen farmland capital construction and regional grain production base and improve the integrated production capacity of land resources of Northwest China.