The relat io n between diurnal variation of leaf net photo synthetic rate ( P_n ) and ecophy siological factors for three years old leaf of Hedera nepalensisvar. sinensis under drought stress was analyzed using CIRAS??2 portable photo synthesis system. T he diurnal variation of H . nep al ensisvar. sinensis photosy nthesis and its midday depression phenomenon was explained clearly and its predominant environmental factors under drought stress were ascertained. Results showed that diurnal variation of H . nep alensis var. sinensis photo synthesis and its micro-environmental factors were connected with soil moisture nearly at the same time. With the agg ravation of drought stress, the daily average and maximum P_n was decreased significantly . Compared with check treatment, the means of P_n in the drought stress of mild treatment, moderate treatment,and severe treatment were reduced by 15%, 47%, and 70%, respectively. With the aggravation of drought stress, the diurnal variation of photo synthesis changed from a single peak curve to a double peak curve and the predominant cause of midday depression turned into non-stomatal limitation. By the methods of stepwise multi??regression and principal component analysis, there was an obvious difference in the environmental factors of impact Pn under different drought stresses. Under the condition of mild soil moisture, the causes of diurnal variation of P_n can be classified into two categories: one was air temperature, relative humidity and CO₂ concent ration and another was photosynthetic active radiation. The effects of photosynthetic active radiation, CO₂ concentration, and relative humidity on P_n became more and more conspicuous with increase in drought stress and relative humidity was the dominant factor.??