Abstract: In this paper the kinetics of fluorescence induction and low temperature(77k) fluorescence excited in chloroplasts, in vivo photosynthetic O₂ release rate and ribulose-1, 5-biphosphate carboxylase in leaves of hybrids and their parents at filling stage were studied. The results indicated that the vigorous hybrids has higher activity and efficieney. The physiological mechanism of heterosis in photosynthesis was diseussed.