The performance of an up flow anaerobic packed-bed reactor in the upgrading of Parkandabad (Mashhad, north east of Iran ) wastewater treatment plant was studied in a pilot plant. The experiments were performed at hydraulic retention times of 6, 12, 18 and 24h based on empty reactor volume and the performance of the reactor was evaluated based on the removal of organic matter (BOD₅ and COD) and SS. The average BOD₅ and COD removal efficiencies were in the ranges of 79.0-89.3% and 75.7-87.2%, respectively, depending on HRT. The relationship between the organic loading rate and organic removal rate was linear in the loading range of 0.52-2.10kg BOD₅/m³.d. The average SS removal efficiencies at hydraulic retention times of 6, 12, 18 and 24h were obtained to be 82.9, 83.6, 81.2 and 87.4%, respectively. The results indicated that the reactor in combination with existing biological treatment process (completely mixed aerated lagoon) can produce a high quality effluent.