The coupling relationship between photosynthesis and transpiration of green leaf is the basis of the coupling relationship between carbon cycle and water cycle in terrestrial ecosystem. Previous studies attribute the most significant cause of the coupling relationship between photosynthesis and transpiration to the stomatal control effect. However, there are few literatures to comprehensively analyze the coupling mechanism between photosynthesis and transpiration. Photosynthesis rate (A_n), transpiration rate (T_r), photosynthetic active radiation (PAR), leaf temperature (T) and stomatal conductance (g_s) were measured in two sunny days from 8:00 to 18:00 for field grown winter wheat (C₃ plant) and summer maize (C₄ plant) in the North China Plain at the stages of anthesis and jointing, respectively. The results show that: 1) the daily variations of PAR, T, g_s, A_n and T_r are similar with each other and the peaks occur around midday; 2) A_n is significantly linearly positive correlated with T_r (the slopes are 1.99 and 3.62 and the correlation coefficients are 0.75^{* *} and 0.92^{* *} for wheat and maize, respectively); 3) both A_n and T_r show positive dependences on PAR, T and g_s; 4) the regression lines of PAR-A_n and PAR-T_r, T-A_n and T-T_r, g_s-A_n and g_s-T_r are similar to each other. In conclusion, our results show: 1) the photosynthesis and transpiration of both wheat and maize during the daily variation are linearly coupled, however their coupling patterns are different, as the slope of A_n-T_r of maize (C₄ plant) is higher than that of wheat (C₃ plant); 2) during daily variance, PAR, T and g_s are simultaneously controlled by solar radiation and show similar daily variations as those of A_n and T_r, this leads to the similar linear correlations of PAR-A_n and PAR-T_r, T-A_n and T-T_r, g_s-A_n and g_s-T_r, and ultimately results in the linear coupling of photosynthesis and transpiration.