Weedy rice WR04-12 and cultivated rice Liaojing 294 were used in field experiment.The results showed that the significant decrease of yield in Liaojing 294 was induced by increasing weedy rice density.The decrease of number of rice panicles and spikelets per panicle,was one of the main reasons causing yield loss of cultivated rice.Milling quality,nutrient quality,and taste quality of cultivated rice decreased with the increasing of weedy rice density,while apparent quality did not show the same trend.The daily minimum temperature and minimum humidity increased in some degree,but the maximum temperature,daily temperature difference,maximum humidity,and daily humidity difference decreased in the cultivated rice population with increasing weedy rice density.The illuminance,at 60 and 90 cm from ground of cultivated rice seems to be decreased with the increase of weedy rice density and that at 30 cm was not influenced by this parameter.As a whole,the weedy rice was a negative effect on micro-ecological environment of cultivated rice population which is the indirect reason of decreasing yield and quality.