We investigated the spherulite growth rate G in 60/40 isotactic polystyrene (i-PS)/atactic polystyrene (a-PS) mixtures under a polarized microscope with a TV video recording system. When the molecular weight M(A) of a-PS was higher than 1.91x10(4), linear growth was observed, i.e. G was constant throughout the crystallization, and the chain diffusivity D-T decreased monotonously with increasing M(A). In contrast, when M(A) was less than 5.2 x 10(3), the growth was non-linear, i.e. G decreased with time, and D, increased with increasing M(A) (D-T proportional to M(A)(1/2)). Both M(A) dependences on D-T were successfully interpreted using a modified Hoffman-Lauritzen theory involving the exclusion effect of a-PS. On the basis of this modified Hoffman-Lauritzen theory, the spherulite growth mode, i.e. linear or non-linear, was also interpreted in terms of a new kinetic parameter lambda = D-A*/D-s,D-a* and D-s being the tracer diffusion coefficient of a-PS and the self-diffusion coefficient of i-PS, respectively.