Abstract: The results of pot experiments in which Hirschmanniella oryzae was inoculated in rice roots in various population densities showed that the population densities of H. oryzae in rice roots were directly proportional to the initial population density inoculated. But the fresh rice plant mass, the fresh root mass, the number of effective tillers, the one thousand-grains weight and the yield of rice were inversely proportional to that of the initial population density of H. oryzae . The injury level of H. oryxae on rice was 15 nematodes in one gram of rice roots at the end of the maximum tillering stage and could be applied as economic threshhold in the control of H. oryxae with several cultivation measures. The yield of rice was significantly was reduced by H. oryxae and was inversely proportional to the population density of H. oryzae in rice roots in the later tillering stage.