This experiment was conducted to find out the critical competitive period and competitive ability of rice against red rice. Plant height and number of tillers of rice decreased as red rice competed with rice during active tillering stage. The maximum tillering stage of rice competed with red rice became about 10days earlier than those non-competed. Significant yield reduction of rice was observed in the plots competed for 20 days after transplating or longer. Such a yield reduction can be mainly attributed to the decrease in number of panicles per hill and grain number per panicle. In a replacement diagram for rice and red rice, their competition is turned out being antagonized. The relative yield total was considered lower than 1. Grain number, culm and panicle length of rice was not affected significantly by inter-specific competition. Rice grain yield reduction was highly correlated with the number of panicles of red rice. Three hundred panicles/$m^2$ of the red rice were necessary to reduce rice grain yield by 50% of the check plot.