【Objective】 The purpose of this study was to obtain high quality raw materials for producing biofuels. 【Method】 Diurnal variation of photosynthetic characteristics of Pennisetum purpureum × P. americanum cv. Reyan No.4 and Miscanthus floridulus was studied and the correlation between net photosynthetic rate and major impact factors was analyzed using the LI-6400 portable photosynthesis analyzer in the field. 【Result and conclusion】 The results showed that diurnal variation of the net photosynthetic rate (Pn) of ‘Reyan No4’ and M. floridulus showed a “single peak”, with no “midday depression” phenomenon. Pn of ‘Reyan No4’ (25.45 μmol·m^-2·s^-1) was slightly higher than that of M. floridulus (25.08 μmol·m^-2·s^-1). Transpiration rate (Tr), water use efficiency (WUE), light use efficiency (LUE) and stomatal conductance (Gs) were similar to Pn and reached the maximum at about 13:00. Tr, LUE and Gs of ‘Reyan No4’ were slightly higher than those of M. floridulus. Diurnal variation of stomatal limitation value (Ls) for both grasses was bell-shaped, reaching the highest value (0.86 μmol·m^-2·s^-1) at about 11:00 and the lowest in the early morning and late afternoon, but diurnal variation of intercellular CO₂ mole fraction (x_i) was the opposite. Pn of two grasses was negatively correlated with atmospheric CO₂ mole fraction (x_a) (P<0.05), and positively correlated with relative air humidity (RH) and Gs (P<0.05), respectively. These results indicate ‘Reyan No4’ has a stronger photosynthetic capacity than M. floridulus.