We determined the effects of ex situ biological wastewater treatment on landfill stabilization under continuous circulation of leachate. Specifically, the waste composition and nitrogen in the leachate during leachate circulation (LC) alone was compared with that in a nitrified leachate circulation (NLC) system. An ex situ sequencing batch reactor (SBR) was applied in the NLC system to oxidize the ammonium to nitrite or nitrate, which was then circulated to landfill for denitrification to nitrogen gas. The chemical oxygen demand (COD) concentration in the leachate was low by NLC versus LC, because in the NLC system, ammonium was oxidized to nitrite/nitrate in the ex situ SBR, and aerobic decomposition and denitrification occurred simultaneously in the landfill, suggesting that the NLC system significantly improves the waste decomposition rate and accelerates landfill stabilization. Because denitrification in the landfill was activated in the NLC system and nitrite/nitrate was reduced to nitrogen gas, the proportion of nitrogen in the gas was higher compared with LC. LC, combined with an SBR, might have value in removing the nitrogen that is discharged from the leachate and in accelerating landfill stabilization, because landfill waste was used as the carbon source for denitrification.