The rice cultivar Erjiufeng grown in pots were used to determine the diurnal changes of photosynthetic rate (Pn) and its major affecting factors, i. e. photosynthetic photon flux (PPF) , temperature, intercellular CO2 concentration (Ci), stomatal conductance (SC), transpiration, and relative humidity (RH). The relation between photosynthetic rate and its factors were analysed by the correlation analysis and path analysis. The PPF is the major factor which affected Pn before 8: 30 am; The Pn reached the maximum values at 8: 30 am, and the midday depression of Pn was caused by the lowering of stomatal conductance. The change of Pn from 14: 30 to 18: 00 were negative correlation with Ci, significantly possitive correlation with PPF, SC, transpiration, leaf temperature and RH.