The utilization of two-line intersubspecific hy-brid vigor is an important way to increase yieldof rice. However, the poor plumpness ofgrains in many combinations is an obstacle forusing the method. We analyzed the relation-ship between the translocation rate of photo-synthetic products into grains and the grainplumpness by the aid of ~(14)C tracer technique.Three hybrid combinations of two-line in-tersubspecies and their male parents were usedin the study. They were Msinchu 53,Huaguang 549, 1611, 2892s/Msinchu 53(F_1), 2892s/Huaguang 549 (F_1), and 2892s/1611 (F_1). Radioactive ~(14)CO_2 was fed to thewhole plant at full-heading (80% heading) for The utilization of two-line intersubspecific hy-brid vigor is an important way to increase yield of rice. However, the poor plumpness of grains in many combinations is an obstacle forusing the method. products into grains and the grain plumpness by the aid of ~ (14) C tracer technique. Three hybrid combinations of two-line in-tersubspecies and their male parents were used in the study. They were Msinchu 53, Huaguang 549, 1611, 2892s / Msinchu 2892s / Huaguang 549 (F_1), and 2892s / 1611 (F_1). Radioactive ~ (14) CO_2 was fed to the white plant at full-heading (80% heading) for