Abstract: The oxygen transfer rate and the performance of both a packed reactor with a natural support of Opuntia imbricata represented as (R1) and a reactor without support represented as (R2) were compared in the treatment of municipal wastewater. The results demonstrated that the dissolved oxygen (DO) and oxygen transfer rate (KLa) were affected positively by air flow rate when increased from 0-2 lpm, mainly in R1. COD removal efficiency was affected by decreasing hydraulic retention time from 30-10 h in both reactors, but highly significant in R2. The R1 reactor showed higher COD removal rate than R2, when the Organic Loading Rate (OLR) was increased from 0.44-1.32 kg COD m 3 day 1. Biomass concentration after 100 day of continuous operation was 2.49 and 12.35 g VSS L 1 for R1 and R2, respectively. The higher oxygen transfer rate, concentration of active biomass and performance of R1 were result of a greater specific surface area of Opuntia imbricata.