The thermal properties and morphology development of isotactic polypropylene (iPP) homopolymer and blended with low molecules weigh atactic polypropylene (aPP) at different isothermal crystallization temperature were studied with differential scanning calorimeter and wide-angle X-ray scattering. The results of DSC show that aPP is local miscible with iPP in the amorphous region and presented a phase transition temperature at T-C = 120 degrees C. However, below this transition temperature, imperfect a-form crystal were obtained and leading to two endotherms. While, above this transition temperature, more perfect alpha- and gamma-form crystals were formed which only a single endotherm was observed. In addition, the results of WAXD indicate that the contents of the gamma-form of iPP remarkably depend both on the aPP content and isothermal crystallization temperature. Pure iPP crystallized was characterized by the appearance of alpha- and gamma-forms coexisting. Moreover, the highest intensity of second peak, i.e. the (0 0 8) of gamma-form coexisting with (0 4 0) of a-form, and crystallinity were obtained for blended with 20% of aPP, the gamma-form content almost disappeared for iPP/aPP blended with 50% aPP content. Therefore, detailed analysis of the WAXD patterns indicates that at small amount aPP lead to increasing the crystallinity of iPP blend, at larger amount aPP, while decreases crystallinity of iPP blends with increasing aPP content. On the other hand, the normalized crystallinity of iPP molecules increases with increasing aPP content. These results describe that the diluent aPP molecular promotes growth rate of iPP because the diluent aPP molecular increases the mobility of iPP and reduces the entanglement between iPP molecules during crystallization. (c) 2005 Elsevier Ltd. All rights reserved.