Abstract:To study effect of timing of topdressing on N2O emission from wheat fields during the period from the jointing stage till the maturing stage of wheat, a field experiment was conducted to have three topdressing schedules (topdressing before rain, topdressing during rain and topdressing after rain) in jointing-maturing stage of wheat from 2007 to 2008. Results show that N2O emission was 37%~67% and 22%~46% less in fields topdressed after rain, than in fields topdressed before rain and during rain, respectively. No significant difference was observed between the treatments in wheat grain yield (p>0.05). Soil water content was the key factor affecting N2O emission during that wheat growing season. Topdressing after rain which brings enough moisture to the soil is an appropriate topdressing method, which may decrease N2O emission significantly in the wheat-growing season with no adverse effect on wheat yield.