Activated sludge is resistant to anaerobic digestion. The cell contents are very degradable but they are protected by the tough cell walls. So the Degradation of VSS of activated sludge in the conventional mesophilic anaerobic process is about 40% at retention times between 30 and 40 days. This research had been constructed to investigate the effect of using pre-selected microorganisms and mixing of municipal and industrial sludge on the performance of biological degradation process and the gas production of the anaerobic digesters. The used Pre-selected microorganisms proved to have a very good effect on the degradation of VSS, TSS, COD, CODS, VFAs and biogas production and even more effective than some chemicals, ultrasound or thermal pretreatments.