Isothermal crystallization rates of isotactic polystyrene from tripropionin (TP), dimethyl sulfoxide (DMSO), and dimethyl phthalate (DMP) solutions were investigated by differential calorimetric measurements. The overall crystallization rate in the DMP solution exhibited a maximum at ca. 70 degrees C, which indicates that the diffusion factor plays an important role in the solution crystallization. The observed temperature dependence of the isothermal crystallization rate was analyzed on the basis of the Fokker-Planck type formula for the diffusion factor. The frictional interaction between the polymer and solvent was estimated, and the result showed that the diffusion processes (molecular motions) in the solution crystallization do not belong to the diffusion limit of Kramers. This fact has been expected for the solution crystallization, in which the frictional interaction between the polymer and solvent is weak. It has also been shown that the above tendency is remarkably dependent on the solvent. The frictional interaction was qualitatively evaluated to be DMP > DMSO > TP.