The relationship between interfacial adhesion and mechanical response in discontinuous glass-fiber-reinforced thermoplastics (GFTP) was investigated using low-density polyethylene (LDPE) as a matrix. It was found that the presence of acid groups in the matrix enhances the mechanical strength as well as the interfacial adhesion, to a great extent, especially with the glass fibers (GF) treated with basic coupling agents. However, some degradation in mechanical properties of the GFTP resulted when excessive interfacial adhesion was achieved with large amounts of the acid groups. The results of diverse characterization suggest the existence of an interphase of bound polymers at the CF surface, whose physical entanglement with the matrix plays a critical role in the mechanical response of the GFTP.