ABSTRACTThe effect of peroxide on the thermal and crystallization properties of high‐density polyethylene (HDPE) and HDPE/polypropylene (PP) blends has been investigated using differential scanning calorimetry and X‐ray diffraction techniques (XRD). Gel permeation chromatography (GPC) was also employed to study the molecular weight distribution (MWD) of HDPE and HDPE/PP blends with or without dicumyl peroxide (DCP). It was observed that blending HDPE with PP increased the rate of crystallization as well as the crystallite size of HDPE, attributed to the lower molecular weight and melt viscosity of the system. A depression of the melting temperature, the degree of crystallinity as well as the rate of crystallization with increased DCP content, confirmed that crystallization was inhibited by the formation of cross‐links and branches in the system. The GPC measurement revealed that the molecular weight and its distribution were influenced by the addition of DCP and/or PP to PE, resulting in the differences in the rate of crystallization, and the size and the degree of perfection of the crystallite.