Zhefu 802 rice isogenic lines IG1, IG4, IG25 and IG26 have the same genetic background but different plant morphology. This paper studied their competition and allelopathy against barnyardgrass. The results showed that IG1 and IG4 had higher plant height, bigger tiller angle and less root vitality than IG25 and IG26, and provided 15% and 17% control of barnyardgrass plant dry weight, which were significantly higher than IG26. The competition ability of test rice isogenic lines against the weed was positively correlated with their plant height and tiller angle, but negatively correlated with their root vitality. The inhibition rate of the water extract of IG1 leaves on barnyardgrass root length was 77.6%, significantly higher than that of non-allelopathic material Xiushui 63, while the IG4 which has the similar plant morphology with IG1 didn't show any significant allelopathy. It seems that there were no correlations between allelopathy and morphological characters of rice under the same genetic background.