Vinyl triethoxysilane grafted high density polyethylene (HDPE-g-YDH151)/Bentonite (BT) composites were prepared via melt compounding and compared with HDPE/BT composites. FTIR proved that HDPE-g-YDH151 is chemically bonded to BT sheets. XRD, SEM and TEM results indicated that the introduction of YDH151 promoted the dispersion and intercalation of BT into HDPE. Consequently, HDPE-g-YDH151/BT composites show satisfied mechanical properties, e.g., the composite with 2phr BT has an elongation at break 29% higher and Young's modulus 32% higher than that of HDPE-g-YDH151. Comparatively, BT aggregated in HDPE/BT composites and all the mechanical properties decreased. Because of high interfacial adhesion between HDPE-g-YDH151 matrix and exfoliated BT, which reduces the mobility of crystallizable PE chain segments, and subsequently reduces the crystallization ability. In comparison, the addition of BT to HDPE did not affect the crystallization behavior of the later.