Novel cyclic sulfoxides, such as 2-octyl, 2-dodecyl and 2-hexadecyltetrahydrothiophene-1-oxide were prepared by the alkylation of tetrahydrothiophene-1-oxide. Additionally, 2-methyl, 2-ethyl and 2-propyl-5-dodecyltetrahydro-thiophene-1-oxide were conducted by further alkylation. Their enhancing activity on the penetration of indomethacin through rabbit skin was evaluated in in vitro experiments, and the effect of the alkyl length on the enhancing activity was discussed. Among the 2-alkyl-tetrahydrothiophene-1-oxides, the compounds containing dodecyl and hexadecyl groups promoted a much greater penetration of the drug through the skin than the compound containing an octyl group. A stronger effect was observed in the experiment using 2-dodecyl-5-alkyltetrahydrothiophene-1-oxide, as compared with the that of 2-dodecyl-tetrahydrothiophene-1-oxide. The substitution of the alkyl groups to the next position of the sulfoxide group seemed to make the enhancing activities large.