Abstract The non-isothermal crystallization behavior and silicate dispersion of high-density polyethylene (HDPE)/modified montmorillonite (20A) composites containing starch masterbatch (starch MB) were investigated by differential scanning calorimetry (DSC), X-ray diffraction (XRD), and transmission electron microscope (TEM). The dispersion of 20A in HDPE matrix depended on the starch content. The Avrami analysis shows that the non-isothermal crystallization process of the HDPE/20A composites followed the Avrami equation with Avrami exponent value in the range of 1.5–2.5. The activation energies calculated by Kissinger method were 350 kJ/mol for HDPE-5, 462 kJ/mol for HD90-5, 542 kJ/mol for HD80-5, and 411 kJ/mol for HD70-5. The activities of nucleation of HD90-5 and HD80-5 were 0.89 and 0.88, respectively but the value of HD70-5 was 0.61. These behaviors can be interpreted by the fact that the increase of the starch MB content enhances the dispersion of clay in HDPE matrix and the good dispersion of 20A affects the degree of super cooling and the nucleation activity of silicate.