Diurnal changes of leaf net photosynthetic rates of two cultivars of Pueraria lobata were measured using the CIRAS-Ⅰ portable photosynthesis system (PP-Systems Co., British), and the relationship among diurnal changes of net photosynthetic rates and those of environmental factors were analyzed. The results showed that diurnal changes in the net photosynthetic rates of both two cultivars could be interpreted as double-peak curves, i.e. they possessed obvious so-called “midday depression” phenomenon. Further analyses showed that the correlation between net photosynthetic rate and stomatal conductance was positive and extremely significant (P＜0.01), the correlations between net photosynthetic rate and intercellular CO2 concentration as well as the transpiration rate were positive and significant (P＜0.05), and the correlations between net photosynthetic rate and air temperature as well as leaf temperature were negative and significant (P＜0.05). The results indicate that among the factors affecting photosynthetic characteristics, some of them could be grouped as stomatal restrictions, while the others could be grouped as non-stomatal restrictions.