Anaerobic co-digestion of tofu liquid waste and rice straw is an impressive study as the mixture of both wastes produced substrates with ideal total solid content. This study aims to analyze the effect of rice straw addition (0,1,3,5 and 7 g) to 250 mL tofu liquid waste on biogas yield. The rice straw addition resulted different variations of total solid which were 1.36, 1.67, 2.28, 2.89 and 3.49% w/w respectively. The increase in total solid content from 1.36 to 2.89% w/w, proportionally increased biogas yield from 58.66 to 130.44 mL/g TS. However, with total solid of 3.49% w/w, biogas yield would be decreased to 78.38 mL/g TS. Furthermore, the more the biogas yield was obtained, the more the total solid was removed. Biogas evolution profile followed the stepped curve and it was successfully modeled through double modified Gompertz models. The predicted optimum TS level to produce the highest amount of biogas yield (239.63 mL/g TS), after mixing with the total solid value of 3.14% w/w was successfully calculated through the Ratkowsky model.