The frequency-dependent viscosity and conductivity of various ionic liquids were measured experimentally, and their mean relaxation times were determined. The relaxation times of the viscosity and conductivity were approximately correlated with their respective zero-frequency limiting values. The Walden products, however, appeared to have no correlation with the ratio of the relaxation time of viscosity to that of conductivity in general. When the alkyl chain of the cation is as short as butyl, more viscous ionic liquids tend to show larger difference between two relaxation times and larger Walden products. Lengthening the alkyl chain of the cation decreases the Walden product while slightly increasing the relaxation time ratio, which was elucidated in terms of the decrease in the high-frequency shear modulus. In addition, the contribution of the mesoscopic structure to viscosity was suggested in the case of the ionic liquid with the longest alkyl chain studied in this work, 1-dodecyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide.