In order to clarify the formation mechanism of polymer spherulites, we have experimentally examined the effect of externally applied gradient field of temperature on the structural evolution of polymer crystallization, especially on the characteristic length scales of the inner structures of nonbanded crystallization of isotactic poly(butene-I) and of banded crystallization of poly(vinylidene fluoride) and polyethylene. The inner structures were strongly influenced by the temperature gradient and suggested the important role of the gradient field of temperature, i.e., of chemical potential in general, at the growth front in the bulk melt. The results are in accordance with our proposal and experimental confirmation on the formation of polymer spherulites based on the instability-driven branching promoted by a self-induced gradient field in the bulk melt.