Abstract: Background and Objective: Utilization of Rice Straw (RS) as an abundant and cheap source of biofuel production is a good example of the second generation of biofuel but its high C/N ratio is one of its disadvantages. This study was aimed to optimize this ratio via synergistic model between Penicillium purpurogenum MM70 and Azotobacter chroococcum. Materials and Methods: Solid state fermentation of RS by P. purpurogenum MM70 and A. chroococcum with the aid of Rock Phosphate (RP), (NH4)2SO4 and Faba Bean Straw (FBS) was used according to Box-Behnken statistical design. The method has been adopted for multi-response surface optimization for biodegradation of RS using the desirability function approach. Design-Expert version 7 was used for both constructing the design and statistical analysis of experiments. Results: Solid state fermentation for nine days was optimal for improving C/N ratio, also for higher production of glucose and hydrolysis of biomass. The optimum experimental design was valid at levels of RP at 250 mg as P2O5, (NH4)2SO4 at 50.0 mg and FBS at 0.90 g per 10 g RS. The obtained responses recorded 35.31±0.21% residual of Fermented Matter (FM), 40.37±1.03 μg–1 FPase and 8.57±0.35 mg of free sugars per g–1 FM., which successfully fermentedto12 g L–1 bioethanol by Saccharomyces cerevisiae Conclusion: Synergistic effect of A. chroococcum with the hydrolytic activity of P. purogenium MM70 is a simple and low-cost method in C/N ratio optimization of RS, production of cellulases and subsequently increase the amount of produced glucose, which can be used in bioethanol production.