Hydraulic retention time (HRT) plays a key role in order to enhance biomass retention at enough density under a suitable organic loading rate (OLR) for maximum organic degradation in an anaerobic digestion (AD) system. The effect of hydraulic retention time (HRT) on volatile fatty acid (VFA) production and organic matter degradation during anaerobic digestion of palm oil mill effluent (POME) was investigated in two anaerobic systems fed with sonicated POME and unsonicated POME respectively. The anaerobic digestion was done at two different HRTs; 10 days and 20 days. HRT and OLR showed vital role in response to the VFA concentration and organic degradation rate. Results showed failure of AD for reactor with HRT 10 days and OLR more than 8.6 mg/L.day due to VFA accumulation and active biomass washout from AD. © 2019 Author(s).