Abstract:The stol)po dominanted by Stipa grandis is one of the zonal type in middle part of Nei Monggol (Inner-Mongolia). Several stands with different biomass and plant height appeared after an accidental fire caused by smoking. On these stands and an additional irrigation treatment, at eomparative study on ph9tosynthetie rate was performed by means of assimilation chamber. The results are as follow: 1. In Jnly, 1981, with little precipitation, the diurnal curve of photosynthetic rate in all stands showed “double peak” form and “midday depression” being pronouneed, while in stands August, it become slight. Particularly, in stands with ample supply of water and fertilizer, the midday depression was ahnost absent. 2. The peak value of photosynthetic rate before noon was diferent from that of afternoon. In arid weather, the peak before noon was higher than in the afternoon, while under rainy weather the reverse was observed. 3. The net photosynthetic rate per day in different stands was different too. the highest one being the stand with sand cover, the next being that with thick soil and burned, and the lowest one being that with thin soil and after burning. All these faets show that the change of and difference in photsynthetie rate are eorrelated with water and nutrient conditions in the environment.