Thermal properties and non‐isothermal crystallization kinetics of polyolefin nanocomposites (high‐density polyethylene/multi‐walled carbon nanotubes) were characterized by differential scanning calorimetry and thermogravimetric analysis. In situ metallocence polymerization was used to prepare nanocomposites of multi‐walled carbon nanotubes (MWCNTs) and high‐density polyethylene (HDPE). This polymerization method consists of attaching a metallocene catalyst complex onto the surface of the MWCNTs followed by surface‐initiated polymerization to generate polymer brushes on the surface. A kinetic equation for the non‐isothermal crystallization was employed to analyze the crystallization characteristics of the nanocomposites. The Avramic exponent, n, can be reasonably well determined from the non‐isothermal crystallization exotherm. The polarized optical microscopy showed that neat polyethylene possessed a well developed spherulite morphology: whereas, the nanocomposites displayed elongated entities that subsequently developed as bundle‐like entities. Non‐isothermal analysis implicitly provides clues about the morphological development history and HDPE molecular ordering around the carbon nanotubes.