Curcumin and calphostin C suppressed the activity and phosphorylation of recombinant UGT1A3 expressed in Sf9 cells. These results indicate that UGT1A3 undergoes phosphorylation, which is required for its catalytic activity. Calphostin C is a highly specific protein kinase C (PKC) inhibitor, so three predicted PKC phosphorylation sites in UGT1A3 were examined. In conclusion, phosphorylation plays an important role in UGT1A3 activity, and the serine at site 43 in UGT1A3 is most likely a phosphorylation site.