Substitution reactions of quinone have been studied for a long time because substituted quinines are known to have important roles in biological activities such as anti-tumor and anti-malarial effects. However, the detailed mechanisms of substitution reactions of quinone are not clearly understood yet. Here, the reactions of 2-methoxy-3-methyl-1,4- naphthoquinone with primary alkylamines are studied. In the reaction, the substitution at the 2-methoxy group yields 2-alkylamino-3-methyl-1,4-naphthoquinone. Each primary amine examined here afforded the substitution product in a high yield, but t-butyl amine failed perhaps due to the steric hindrance.