To determine the optimum rate of urease inhibitor(UI),a field experiment was conducted to investigate the effects of UI on nitrogen transformation in paddy fields.Addition of urease inhibitor N(nbutyl)thiophosphoric triamide \[NBPT\] was designed at five rates of 0.5%,0.75%,1.0%,1.25% and 1.5% of urea,respectively.The results showed that urea added with 1.0%,1.25% and 1.5% NBPT notably increased grain yields and N efficiency,and there was no significant difference between the 1.0% and 1.5% treatments,while addition of 0.5% and 0.75% NBPT had no significance effects on them.When the rate of NBPT in urea was more than 1.0%,the urease activity and the content of NH+4N in soil were obviously reduced in tillering stage,while the content of NH+4N in soil was increased in booting stage,however there was no obvious effect on nitrate reductase activity(NRA),soil NO-3N content,MBC and MBN in both stages.Then,there was no significant difference in urease activity between with and without NBPT at booting stage.The stepwise regression analysis revealed that the grain yield of rice was significantly associated with NH+4N content in soil at the tillering and booting stages,especially,the latter.However,other properties in soil had no obvious effects on the grain yield.NBPT slowed down the hydrolytic action of urea and dramatically improved soil NH+4N content in the booting stage,which appeared to be the dominate factor of increasing yields and N efficiency.The optimum rate of NBPT in urea should be above 1.0% in paddy fields.