Thermophilic anaerobicdigestionofwaste activated sludge (WAS)was experimentallystudied in this research.WAS using cattle dung inoculumswith total solids (TS) concentrations of 12.02, 17.58, 23.28, 26.75, and 35.2 g L-1were digested anaerobically in a batch digester at thermophilic temperatures (55C°) for a retention period of 13days. Effect of TS concentration on the quality and quantity of the produced gas, pHvariation,andthekineticsofbiogas productionwere investigated. The results showed that biogas production potential and biogas production rate increasedwith an increasingTS concentration.Themaximumbiogas yields from TS concentration 12.02, 17.58, 23.28, 26.75, and 35.2 g L-1 were 0.186, 0.189, 0.93, 0.213, and 0.231L(gVS)-1, respectively.ModifiedGompertz equationwas employed tomodel the biogas produc-tion at different substrate concentrations.The equation gave agoodapproximationof themaximumbiogasproduction (Rm) and the biogas yield potential (P)withcorrelation coefficient (R2) over 0.992. The digestion at TS concentration 35, 8 gL-1 gave the best results. The maximumbiogas production reaches0.856Lday-1,and the biogas yieldwas 6.650 Lat the end of the 13th day of the experiment.This amount of biogaswith composition 72.59%of CH4, and 23.6%of CO2 is equivalent to 190KWh of electricity. These results showthatWASmixedwithcowdungis an effective feedstockfor biogas production, givinga high cumulativebiogasyield.