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The ability of four Aspergillus strains for biosynthesis of kojic acid was evaluated among which Aspergillus terreus represented the highest level (2.21g/L) of kojic acid production. Improvement kojic acid production ability of A. terreus by random mutagenesis using different exposure time to ultraviolet light (5-40min) was then performed to obtain a suitable mutant of kojic acid production (designated as C 5-10 , 7.63g/L). Thereafter, design of experiment protocol was employed to find medium components (glucose, yeast extract, KH 2 PO 4 (NH 4 ) 2 SO 4 , and pH) influences on kojic acid production by the C 5-10 mutant. A 2 5-1 fractional factorial design augmented to central composite design showed that glucose, yeast extract, and KH 2 PO 4 were the most considerable factors within the tested levels (p<0.05). The optimum medium composition for the kojic acid production by the C 5-10 mutant was found to be glucose, 98.4g/L; yeast extract, 1.0g/L; and KH 2 PO 4 , 10.3mM which was theoretically able to produce 120.2g/L of kojic acid based on the obtained response surface model for medium optimization. Using these medium compositions an experimental maximum Kojic acid production (109.0±10g/L) was acquired which verified the efficiency of the applied method., (Copyright © 2018 Sociedade Brasileira de Microbiologia. Published by Elsevier Editora Ltda. All rights reserved.)