This article presents the optimization of gallic acid production using filamentous fungi from tannin-rich mixed substrates taking into account the interaction effects of six variable process parameters. The methodology adopted for optimization was the evolutionary operation (EVOP)-factorial design technique. This technique combines the factorial method for designing experiments with the EVOP methodology for analyzing the experimental results systematically and arriving at conclusions according to its decision-making procedure. Standard deviation and error limits based on 95% confidence were calculated according to the relationship given in the literature. It was found that the best combinations of the process parameters at the optimum levels were 30degreesC, 80% relative humidity, pH 5.0, 48-h incubation period, 3 mL of induced inoculum, and 35 g of mixed substrate, resulting in a gallic acid yield of 94.8% under modified solid-state fermentation.