In the melt-mixing process of high density polyethylene (HDPE) and glass fibers (GF), four types of composites with various interfacial bond strength were obtained by adding maleic low molecular weight polyethylene (MPEW) or maleic anhydride (MAH) and initiator, etc. The mechanical properties of these composites and their dependence on the matrix crystalline morphology were investigated by scanning electron microscopy, small-angle light scattering, differential scanning calorimetry, wide-angle X-ray diffraction, and a material universal mechanical testing machine. The occurrence of the interfacial transition regions made of extended-chain crystals around glass fibers was found to be the result of crystallization effects induced by the interfacial stress. The interfacial stress was mainly produced from the matrix shrinkage in the specimen molding process. Under high interfacial bond strength conditions, the forming process of the extended-chain crystals was found to both relax the interfacial stress and at the same time, enhance the interfacial phase modulus and improve the mechanical properties of the composites. Under a 30% glass fiber content condition, the extended-chain crystals formed along the normal direction of glass fiber surfaces connected with each other, fully filled the matrix, and led to a significant increase in the Charpy impact strength of the composites. By contrast, under weak interfacial adhesion, the interfacial stress was released by a dewetting process of the interfacial phase and by the formation of interfacial cracks. Consequently, the interfacial stress did not influence the growth of the spherulites in the matrix and at the same time, the Charpy impact strength of the composite was lower.