Untreated and treated with alkali (NaOH), acrylic acid (AA), diammonium phosphate (DAP), and maleic anhydride (MA) of coconut filter (CF) fibers were characterized using scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), thermogravimetric analysis (TGA), and single fiber test. The composites were fabricated from CF and film resins (Polylactic-acid (PLA) and Polypropylene (PP)) using a hot press machine. Generally, our results indicated that chemical treatments improved the mechanical properties of CF fiber composites, except for the DAP-treated fiber/PP composite. The AA treatment of fiber produced the best adhesion at the fiber-matrix interface. Consequently, the tensile and flexural strengths of AA-treated fiber-reinforced polymer were the highest. CF fiber-reinforced PLA composites had better mechanical properties than CF fiber-reinforced PP composites. Our results show that CF fiber is feasible as a reinforcement for polymer composites.