Abstract: To investigate the effect of flooding disturbance on the net primary productivity of Songnen steppe,a comparatively thorough study was conducted on Sanjiadian State owned Rangeland in Da'an city,Jilin Province,which was partly flooded in1998. The study site was located in the south Songnen plain of Northeastern China,dominated by Leymus chinensis grassland. An extensively mild slope with flooding gradients (from un flooded to heavily flooded) was taken as the study site. Two flooded transects coded FLand FHwhich was respectively subjected to 3 and 9 months of flooding were designed,and an un flooded one coded CKat a relatively higher elevation was set as a control. Before flooding occurred in1998,the slope had an almost uniform soil and L.chinensis dominated vegetation. Each transect was 0.2 hm² (100 m?20 m) in size,and the two flooded transects were almost paralleled each other,with the longer sides of them perpendicular to the retrieving direction of floodwater. In each transect,twenty 1 m² sized quadrats were randomly chosen to survey the community structure and the aboveground biomass. Comparative analyses were made on the dynamics of soil water,soil Nand P,and species composition of grassland communities that occurred in responses to flooding disturbance. The results showed that the lightly and heavily flooded transects had a significantly larger aboveground biomass than the control,with the increase of 89.54% and 113.45%,respectively. The heavily flooded transect had a slightly but insignificantly larger aboveground biomass than the lightly flooded one,indicating that on flooded sites,water was not the limiting factor of the aboveground biomass. The acute changes of soil water caused by flooding led to the changes of soil nutrients and species assemblages,which would impact community biomass. Just as the case for aboveground biomass,the soil water contents of the two flooded transects were significantly larger than that of control,which was the direct effect of flooding disturbance. The contents of soil available Nand Pon the flooded transects were higher than that on the control,as resulted by the increase of soil water. The species composition had an obvious difference between flooded transects and control,and the major change on the flooded transects was the decrease of Xeric and Mesoxeric plants and the increase of Hygric and Hygro mesic plants,compared to the control. It's deduced from the results that the mechanisms that the flooding disturbance influenced the net primary productivity were: 1) Flooding disturbance firstly changed soil water content,the leading limiting factor of grassland productivity,especially in dry years,and thus,lessened the limitation of aridity on grassland productivity. 2) Flooding induced increase of soil water content led to the drastic increase of nutrient availability,and thereby,eliminated the restriction of pre flood nutrient scarcity on grassland productivity. 3) Still due to the afterward effects of flooding disturbance,the species composition of the community transformed from relatively shorter plant species assemblage towards taller and bigger plant species assemblage that were more productive.