Abstract A rectangular photo-reactor equipped with carbon black (CB)/ZnO nanocomposite film was used for photodegradation of methyl orange. Central composite design was used for evaluation of the effects of initial dye concentration, reaction time, CB/ZnO ratio and initial pH. The high correlation coefficients (R2=0.954 and adjusted R2=0.912) obtained by analysis of variance (ANOVA) demonstrated close fit between the predicted and experimental values. The initial dye concentration, reaction time, CB/ZnO ratio and initial pH were 13mg/L, 95min, 0.05 and 5, respectively for 80% decolorization efficiency. The result of TOC analysis showed that the process allowed 55.79% mineralization under optimized conditions.