A thioamide, N-5-trifluoromethyl-6-methoxy-1-thionaphthoyl-N-m ethylglycine, undergoes trans${ ightleftharpoons}$cis photoisomerization around C-N bound in solution. Azulene quenching studies showed the photoisomerization to proceed via both singlet and triplet excited states.The total quantum yield of the trans${ ightarrow}$cis photoisomerization is about 0.26, 0.14 from the singlet excited state and 0.12 from the triplet excited state. Intersystem crossing and internal conversion quantum yields were calculated from sensitized photostationary state and a plausible mechanism is proposed.