Thermophilic aerobic digestion of raw sludge was studied and optimized at bench scale. Experimental studies were conducted using two bench scale systems. The first was an insulated closed stainless-steel vessel of 21 volume. The second digestion system consisted of three closed stainless-steel vessels of 51 volume each. 50% and 70% reduction in dry residue (D.R.) and volatile solids (V.S.) respectively were achieved at a temperature of 50-degrees-C, 0.5 m3 m-3 min-1 air flow rate, and 7 days hydraulic retention time, using a mixed sludge of 3.5 to 6% dry solids. Kinetic parameter constants were found to be greatly affected by reaction temperature and air flow rate. Reaction rate constants were found to be 0.076, 0.15 and 0.21 day-1 at 40, 52.5 and 58.5-degrees-C, respectively. Activation energy was determined and found to be 11.23 kcal.