Rice straw is one of lignocellulosic biomass with abundant resources in Ethiopia and a suitable feedstock for biogas generation. Due to the recalcitrant nature of these biomass pretreatment step was performed. The rice straw treated with sodium hydroxide was used as a main feedstock for solid-state fermentation experiments. The treated rice straw biomass was mixed and digested with food waste at different mixing ratio. The fermentation process was initiated using cow dung as inoculum for biogas production in solid-state digestion system at 20 % of TS. The effect of the temperature inside the digester (ambient, 35C, and 55C), substrate to inoculum ratio (0.5, 1, and 1.5), and amount of food waste (0, 25, and 50 %) in feedstock on biogas production was investigated by central composite face centered experimental design. The highest biogas production of 1300 ml (1.29 ml of biogas per gm of biomass) was obtained at the substrate to inoculum ratio of 1, 25 % of food waste in the feedstock at operating temperature of 55C. The conditions that maximized the biogas production were a substrate to inoculum ratio of 1.2, food waste amount of 29.75% that would operate at a temperature of 55 ℃ which results 1254.03 ml of biogas. The methane content of selected experimental runs was determined and it ranges from 30.8 to 38.9 %.