This study evaluated the effectiveness of sewage pretreatment as it affected the subsequent anaerobic digestion of waste activated sludge (WAS). The key to this sludge pretreatment process was that microorganism cells within WAS were ruptured by mechanical jet and smash under pressurized conditions, 5–50 bar pressure. Cell rupture was estimated by soluble protein concentration (SPC) because the cytoplasm of microorganisms is mainly composed of protein. The SPC in the WAS varied significantly before and after pretreatment. In particular, cell rupture increased from 6% to 43% with the increase of the pretreated pressure (5–30bar), volatile solids concentration and thickening time of WAS. In the WAS pretreatment with long-thickening time (12h < thickening time ≤ 48h), cell rupture was monitored 5–10 % higher than short-thickening time (Oh <thickening time ≤ 12h). Generally, it was observed that higher anaerobic digestion efficiencies of WAS could be obtained according to the increase of microorganism cell rupture through mechanical pretreatment of WAS.