In this study, we have investigated the hypo-pigmentary mechanism of methanol extract of Forsythiae Fructus in human melanocyte cell line, HM3KO. Treatment of HM3KO cells with Forsythiae Fructus extract markedly inhibited melanin biosynthesis in a dose-dependent manner. Decreased melanin contents occurred through the decrease of tyrosinase protein and activity. The mRNA levels of tyrosinase and tyrosinase-related protein 1 (TRP-1) were also reduced by Forsythiae Fructus extract. Moreover, the level of intracellular cyclic AMP (cAMP) was significantly decreased by treatment of Forsythiae Fructus extract. These results suggest that Forsythiae Fructus reduces melanin synthesis by down regulation of tyrosinase mRNA transcription, and this is closely related to the cAMP-dependent pathway.