In this study, a series of organic/inorganic composites were fabricated by melt-mixing ultra-high molecular weight polyethylene (UHMWPE) powder with a mixture of magnesia, potassium di-hydrogen phosphate and water. The microstructure and chemical composition of the composites were characterized by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). Three point bending tests showed that the flexural strength and toughness of the composites were improved with the increase of polymer content. The flexural strength of the composite with 25.0 wt% UHMWPE was 7.7 MPa, and can be dramatically enhanced to 21.1 MPa by incorporating only 0.1 vol% of oriented thermotropic liquid crystalline copolyester (TLCP) fibers. The improvement is due to the enhanced interfacial bonding between the polymer and fibers, which was much stronger than that between the inorganic phase and fibers, making the fibers adequately contribute to the mechanical strength of the three-component composite. Our results demonstrated that the interfacial strengthening between fiber and inorganic matrix achieved by interpenetrated organic phase helps to establish a strong composite system.