Livestock wastewater has been known as one of recalcitrant wastewaters, which contains high concentration organics, suspended solids and ammonia nitrogen. This study was undertaken to improve the biodegradability of livestock wastewater by a gamma ray irradiation, and finally to improve the biological nitrification efficiency using a biological nitrification process. As the absorbed dose of gamma ray was increased, the biodegradability of wastewater increased and reached the highest value at 10 kGy of gamma ray dose. The efficiencies of ammonia nitrogen removal and nitrite/nitrate production were also reached the highest values at 10 kGy.