Abstract: In 2007, the results of a long-term fertilizer experiment were used to study the characteristics of weed communities in a paddy field, where crop straw was returned and organic fertilizer applied since 1985 (Jinxian County, Jiangxi Province). Weed species biodiversity was significantly reduced under long-term straw return and organic fertilizer application. Long-term straw return through mulching reduced weed density and biomass, but they were increased during the field fallow by organic fertilizer application. The positive effects of organic fertilizer application on weed growth were greater in early planted rice plots than in late planted ones. The negative effects of straw return on weed growth were greater in winter with mulching after application than in summer when straw was mixed into the soil. Alopecurus japonicus was the dominant population with a relative density of more than 90% in all plots and the next common weed populations were Lapsana apogonoides and Aneilema keisak. This suggests that straw return and organic fertilizer application can effectively control weed communities in the paddy field during fallow.