Molecular weight dependence of growth and morphology of spherulites of isotactic poly(butene-1), iPB-1, and those of the mixtures with atactic poly(butene-1), aPB-1, were examined by atomic force microscopy (AFM) and polarizing optical microscopy (POM) in order to examine the mechanism of the structural evolution by the branching and re-orientation of lamellar crystals at the growth front. The width of lamellar crystals and the characteristic size of the inner structure of spherulites decreased with increasing molecular weight. The result suggests that the mobility of the melt determines the sizes in spherulites and supports the growth front instability induced by a gradient triggering the branching. The sizes in the mixtures also decreased with increasing weight-averaged molecular weight. M-w. The size dependence in low M-w region, however, was too strong and that in high M-w was too weak in comparison with the predicted dependence for the prepared M-w. It has been concluded that the peculiar behaviors should be discussed with effective M-w influenced by the occurrence of separation and exclusion of non-crystallizing aPB-1 at the growth front. (C) 2011 Elsevier Ltd. All rights reserved.