Ammonia volatilization in relation to N application rate and climate factors was studies through field experiment in red soil under the crop rotat ion of Smooth Crabgrass (Digitaria ischaemum) in spring and Winter Radish (Raphanus sativus) in autumn.Results indicate that ammonia volatilization in Treatments N90, N160 and N230 in spring dragged on for 10~17 days, and peaked at the 6th~8th day after urea application, reaching N 0.11, 0.57 and 1.84 kg hm^-2d^-1, respectively, and the total amount of N loss through ammonia volatilization was N 0.67~5.16 kg hm^-2, accounting for 0.74%~2.24% of the total N applied In autumn, in Treatments N70, N130, N190 and N250 (the ratio of basal and sidedressing is 7.3) ammonia volat ilization occurred both after application of basal fertilizer and tillering fertilizer and lasted 10~11 days, with peaks reaching N 0.02, 0.05, 0.06 and 0.09 kg hm^-2d^-1 for the basal fertilizer, and N 0.05, 0.22, 0.38 and 0.72 kg hm^-2d^-1 for the tillering fer tilizer respect ively.The total amount of N loss through ammonia volatilization was N 0.37~3.04 kg hm^-2, accounting for 1.31%~3.69% of the total N applied to Winter Radish (Raphanus sativus).