In order to investigate effects of different barnyardgrass species on growth and yiled of rice, two rice cultivars, Xinliangyou 6 hao (an indica hybrid cultivar) and Nanjing 46 (a japonica cultivar), were cocultured with four barnyardgrass species grown at a density of six plants·m^-2 from 10 days after transplanting to maturity. The treatments were designed as follow: weed free (control), rice with Echinochloa crusgalli var. mitis  (T₁), rice with E. crusgalli (T₂), rice with E. crusgali var. zelayensis (T₃), and rice with E. colonum (T₄). The result showed that barnyardgrassinduced reductions in grain yield of rice were obviously different among the four treatments. T₁, T₂ and T₃ treatments reduced the grain yield of indicia cultivar by 19.2%, 10.8% and 21.9%, and the respective reductions in japonica cultivar were 39.7%, 25.3% and 47.3%, respectively. However, no significant difference was detected for T₄ treatment. During rice coculture with barnyardgrass, T₁, T₂ and T₃ significantly reduced rice dry matter accumulation at maturity, flag leaf photosynthetic rate, root oxidation activity and the activity of adenosine triphosphate enzyme (ATPsse) in rice grains at the filling stage, and the magnitude of decrease was in the order of T₃> T₁ >T₂, while no significant difference was observed between T₄ and CK. In addition, all treatments had no effects on the final tiller number and plant height of rice. It indicated that the negative effect of barnyardgrass on rice growth and yield differed among the four species of barnyardgrass, in the order of T₃>T₁>T₂>T₄. Barnyardgrass reduced the flag leaf photosynthetic rate of rice, both root oxidation activity and ATPsse activity in grains, which resulted in the reduction in final productivity of rice when cocultured with barnyardgrass.