Three japonica rice cultivars,Mudanjiang 30,Dongnong 426,Songjing 14,were used as materials to study the effect of N fertilizer application and transplanting density on grain transaminase activity,protein content of grain,protein yield and yield of japonica rice in cold region by splitplot design.The results were as follows:The activities of glutamate oxaloacetate transaminase(GOT) and glutamate pyruvate transaminase(GPT) in three cultivar grains showed a single curve in heading stage and reached the peak at different time; Besides the GPT activity in 14 days after heading,the activity of GOT and GPT in 21 days,28 days,and 35 days after heading had significantly or very significantly positive correlation with grain protein content during 21 days,28 days,and 35 days after heading; Grain protein content improved with increasing of N application,while there were not obvious difference among the density treatments; Considering the yield and protein yield,the best treatment for Mudanjiang 30,Dongnong 426,and Songjing 14 were N2D3(N 150 kg/hm2,30 cm×16.6 cm),N3D2(N 225 kg/hm2,30 cm×13.3 cm),N3D1(N 225 kg/hm2,30 cm×10.0 cm),respectively.