Slaughterhouse wastewater (SWW) is a typical source of pollution and creates serious environmental concerns. Therefore, SWW needs a cost effective and efficient treatment technology for safe discharge and consequently for public health protection. The aim of the study was to evaluate the treatment of SWW by two steps, first one was primary treatment by screening and settling followed by biological treatment using of Horizontal Flow Biofilm Reactor (HFBR) as a second step. A significant review of raw SWW characteristics was presented, COD, BOD and TSS concentrations values were very high its maximum values reached 9855, 5037 and 3440 mg/l, respectively. The primary treatment removed 34% COD and 75% TSS. The performance of HFBR system has been investigated in continuous mode. The system was operated at two different organic loading rates to find out the optimum operating conditions, which produce treated effluent characterization compatible with the Egyptian legislation for discharge on sewerage network. The organic loads (OLR) applied to the HFBR system were 0.53 and 1.3 kg COD/m2/d. During the steady-state of optimum operating conditions, the system achieved significant carbon removal of 85% and 95% for COD and TSS respectively. Nitrogen removal percentage reached 50% ammonia-nitrogen and 56% for total nitrogen. From the results obtained HFBR could be considered as a reliable treatment technology for SWW.