Within the scope of experiments, five kinds of biaxial weft-knitted (BWK) fabrics with various loop lengths (8.0, 9.2, 10.5, 11.9, and 13.5 mm) were used as reinforcement systems to fabricate thermoplastic composites with polypropylene (PP) resin yarn. Then, the final composite became BWK composite with various loop lengths. The mechanical properties of the composites were investigated by conducting tensile, three-point bending, and three-point bending impact tests on specimens. In all specimens, PP was commingled with glass yarn. Glass was used as the reinforcement material. Fiber volume fraction of weft fibers with the 8.0 mm loop length was the highest compared with the other four types of specimens. Because of the higher volume fraction of the BWK composites with the 8.0 mm loop lengths, tensile, three-point bending, and three-point bending impact properties of the 8.0 mm loop were higher than the other four types (9.2, 10.5, 11.9, and 13.5 mm) of composite structures.