A series of high density polyethylene (HDPE)/ CaCO3 blends were prepared with different kinds of coupling agents, with CaCO3 particles of different sizes, and with matrixes of different molecular weights during the melt-mixing of HDPE and CaCO3 particles. The mechanical properties of these blends and their dependence on the interfacial adhesion and matrix crystalline structure were studied. The results showed that the Charpy notched impact strength of these blends could be significantly improved with an increase in the interfacial adhesion or matrix molecular weight or a decrease in the CaCO3 particle size. When a CaCO3 surface was treated with a compounded coupling agent, the impact strength of the HDPE/CaCO3(60/40) blend was 62.0 kJ/m(2) 2.3 times higher than that of unimproved HDPE; its Young's modulus was 2070 MPa, 1.07 times higher than that of unimproved HDPE. The heat distortion temperature of this blend was also obviously improved. The improvement of the mechanical properties and the occurrence of the brittle-tough transition of these blends were the results of a crystallization effect induced by the interfacial stress. When the interfacial adhesion was higher and the CaCO3 content was greater than 30%, the interfacial stress produced from matrix shrinkage in the blend molding process could strain-induce crystallization of the matrix, leading to an increase in the matrix crystallinity and the formation of an extended-chain (or microfibrillar) crystal network. The increase in the critical ligament thickness with an increasing matrix molecular weight was attributed to the strain-induced areas becoming wider, the extended-chain crystal layers becoming thicker, and the interparticle distance that formed the extended-chain crystal network structure becoming larger with a higher matrix molecular weight. The formation of the extended-chain crystal network and the increase in the matrix crystallinity were also the main reasons that Young's modulus and the heat distortion temperature of this blend were improved. (C) 2003 Wiley Periodicals, Inc.