This study was carried out to treat the thermal elutriated acids of piggery wastewater using UASB process. The UASB reactor was operated at an organic loading rate (OLR) of 7.4 kgCOD/m3-day (6.5~9.0). During the start-up period, the low COD removal efficiency (20%) was caused by shock loading and instability in the reactor. It was mainly due to the high concentration amounts of ammonia nitrogen, which caused inhibitory and toxic effects to toward the anaerobic bacteria. In steady state, the UASB reactor showed a SCOD removal efficiency of 71% and a VS removal efficiency of 39%. The gas production and methane content were 1.3 L/day (0.21 m3 CH4/kg COD removed) and 77%, respectively.