ABSTRACT The ability of four Aspergillus strains for biosynthesis of kojic acid was evaluated among which Aspergillus terreus represented the highest level (2.21 g/L) of kojic acid production. Improvement kojic acid production ability of A. terreus by random mutagenesis using different exposure time to ultraviolet light (5-40 min) was then performed to obtain a suitable mutant of kojic acid production (designated as C5-10, 7.63 g/L). Thereafter, design of experiment protocol was employed to find medium components (glucose, yeast extract, KH2PO4 (NH4)2SO4, and pH) influences on kojic acid production by the C5-10 mutant. A 25-1 fractional factorial design augmented to central composite design showed that glucose, yeast extract, and KH2PO4 were the most considerable factors within the tested levels (p < 0.05). The optimum medium composition for the kojic acid production by the C5-10 mutant was found to be glucose, 98.4 g/L; yeast extract, 1.0 g/L; and KH2PO4, 10.3 mM which was theoretically able to produce 120.2 g/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 ± 10 g/L) was acquired which verified the efficiency of the applied method.