A novel hybrid biological reactor which contained both suspended- and attached-growth biomass was developed by introducing porous materials into a regular activated sludge unit and used for the treatment of domestic wastewater. The development of suspended- and attached-growth biomass in this reactor and the effect of the organic loading rates (OLR) on the operational performance were investigated. SEM observation revealed three different patterns of biomass immobilization in carriers, micro-granules mechanically retaining in the polyurethane foam pores, small multi-cellular structures attaching to the inner polyurethane surface and individual microbial cells attaching on the polyurethane surface. The total biomass concentration in hybrid reactors increased to 4.30-5.75 g/l when the volumetric portion of the carrier was 15-30%. The concentration of attached-biomass increased with increase of OLR, moreover, the biomass fixed in the carriers predominated in the reactor at all OLR applied in this study, suggesting that the attached-biomass played a major role in COD removal.