Duckweed is one of the key hydrophyte in floodwater which may effect N transformation in paddy field.It studied the influence of duckweed in different density on N conversion in floodwater after urea application through greenhouse culture.Soil was colleted from Shuangqiao farm,Zhejiang province,which was one of the typical paddy soil in Hangjiahu plain.The results showed that duckweed would accelerate urea hydrolization and the velocity constant of urea hydrolization in control,low-density duckweed(D_1) and high-density duckweed(D_2) treatments were 0.02,0.03 and 0.04/h,respectively.In the first days of the experiment,duckweed may uptake and assimilate large amount of NH~ _4-N and then save them,however,from the 10~(th) day(in D_2 treatment) and the 12~(th) day(in D_1 treatment),part of N was released from duckweed and then NH~ _4-N and NO~-_3-N in floodwater increased obviously.It was indicated that duckweed could save part of N when large amount of N was applied into paddy field while release N when N concentration in floodwater was very low,which was helpful for reducing N rainfall potential and providing N nutrient for paddy rice persistently.