This study was focused on the capacity investigation of a novel multistage flexible fiber biofilm reactor (MS-FFBR) to treat milk processing wastewater (MPW) with high organic loading (OLR). The MS-FFBR performance was evaluated at four intermediate stages separately and also the final effluent quality of the overall system with an influent chemical oxygen demand (CODin) ranged from 1500±20 to 6000±50 mg/L and hydraulic retention times (HRTs) of 8, 12, 16 h. By comparting the bioreactors into the four stages effectively enhanced the bioreactor's performance. The maximum TCOD removal efficiency was achieved at the first stage, which was about 89±20, 82±20 and 78±20 % at HRTs of 16, 12, 8 h, and low CODin of 1600±20, 1590±20, and 1673±20 mg/L, respectively. However, the first stage had less contribution to TCOD removal at high CODin concentrations, reported to be about 42±4 %, 46±4 %, and 25±4 % at CODin of 5960±40, 5830±40, and 5870±40 mg/L, respectively. Furthermore, the MS-FFBR was effective in removing total suspended solids (TSS) and turbidity. The bioreactor has reduced the effluent turbidity to 9.0±0.2, 20.0±0.6 and 16.1±0.5 NTU at low CODin concentrations of 1600±20, 1590±20, and 1670±20 mg/L and HRTs of 16, 12 and 8 h, respectively. The bioreactor revealed a high COD removal rate increased from 2.3±0.1 to 12.2±0.4 kg TCOD/m3d by increasing the OLR from 2.4±0.1 to 17.6±0.4 kg TCOD/m3d, confirming high reactor capacity for treatment of high strength wastewater. Kinetic studies confirmed that the biomass yield was low at various HRTs ranging 0.1-0.2 gVSS/gCOD.