High solid anaerobic digestion (HSAD) for sewage sludge is a promising option to conventional AD since it minimizes treatment volumes, costs of transportation, and lower requirements energy; however, the long duration of the digestion is a major drawback. The purpose of this research was to study the effect of high solid anaerobic digestion of domestic WWTPs sludge on Methane yields and biochemical mechanisms. The effects of high-thermal pretreatment and solid content were examined in the batch bioreactor for 22 days to determine the optimal thermal pretreatment temperature and the best solid content, the results show clearly that the TH improve the digestion of HSAD as the solid content ranged from 10 to 16 % with pretreatment at 180 ℃; the removal ratio was increased by 12.80% to 38%, 44% to 74% and 23% to 50% for COD, TS and VS respectively; also the gas production increased by 28 to 50% while methane yields were not significantly different, the solid content of 11% gives the high biogas production. Also, for sludge with a solids content of 11% at different temperatures the pretreatment at 180 ℃ gives high biogas production. and improved the performance as the COD, VS, and TS removal ratio increased from 49.28%, 30.91 %, and 36.45% to 71.68 %, 52.73%, and 63.5 % respectively, and the biogas production increased.