CH3I in a supersonic jet was isolated in a low-temperature argon matrix. The observed infrared bands were assigned to monomer and dimers by examining dependence of the intensities on the mixing ratio with argon gas and the stagnation pressure of the supersonic jet. Ab initio calculations at the MP2 level predicted that head-to-tail (HT) and head-to-head (HH) structures of CH3I dimers were stable. The dimerization energy of HT obtained by a single-point-energy calculation at the MP4(SDTQ) level was 185 cm(-1) larger than that of HH. A comparison of the observed and calculated wavenumbers evidences that HT and HH co-exist in the jet.