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Ultraviolet (UV) irradiation was used to induce mutagenesis in Aspergillus niger to provide a suitable mutant strain for overproduction of cellulase. Response surface methodology was successfully utilized to evaluate the effect of the mutation conditions, namely: UV exposure time (60–300 s) and distance of the strain from the UV source (0–20 cm) on created clear zone area around the strains. A maximum clear zone area (600.525±15.537 mm2) was obtained under the optimal mutation conditions of UV exposure time and distance of the 9 cm and 220 s. Rice and wheat straw were used as cellulosic substrates to produce cellulase by using the mutant strain through solid state fermentation. The results indicated that the total cellulase activity of the produced cellulase during 10 days of fermentation was 4.159 IU/ml, which was approximately twofold higher than that of the cellulase for the wild strain.