Morphology development and thermal properties of polyolefin composites (tungsten oxide (WO3)/ high density polyethylene (HDPE)) were characterized by differential scanning calorimetry, polarized optical microscopy, and nonisothermal crystallization kinetics. Tungsten oxide (WO3) and HDPE composites were prepared by the in-situ metallocene polymerization method which consisted of attaching a metallocene catalyst complex onto the surface of the nuclei (WO3) and followed by the surface–initiated polymerization. A kinetic equation proposed by Seo was employed to analyze the nonisothermal crystallization characteristics of the composites. The polarized optical microscopy and the Avrami exponent verified the importance of the interaction between the HDPE molecules and the nuclei surface for the HDPE molecular ordering around the metal powder as well as the morphological development in the early stage. The Avrami exponent, n, determined from the nonisothermal crystallization kinetics analysis indicates random 3-dimentsional morphology development for the WO3/HDPE composites. Dispersed WO3 accelerated the crystallization rate due to heterogeneous nucleation effect as indicated by the shift in the crystallization peaks to higher temperatures. The obtained crystalline structures were compared to those for the neat polyethylene resins and carbon nanotube (CNT)/ HDPE composites.