Abstract: This study examines the natural Leymus chinensis grassland in Northeast China. An object of study is Leymus chinensis population which is constructive population. The study results shows that the diurnal change of photo-synthetic rate of Leymus chinensis population was a curve of double peak. The first peak value appeared around ten o'clock in the morning, which were 2.64g CO₂/m² (ground)·hr and second peak value appeared around fifteen o'clock in the afternoon,which were 2.29g CO₂/m² (ground)·hr. The total net photosyn-thetic rate were 25.74g CO₂/m² (ground)·d. There are close relationships between the diurnal change in the photosynthetic rate of Leymus chinensis population and integ-rated effects of environmental factors. The limiting factors influencing the reduction in the photosynthetic rate at the noontime are the lower air relative humidity and soil moisture content.