When cellulose fibers are ethanol pretreated followed by plasma treatment, their surfaces become more hydrophobic, resulting in better interfacial adhesion to hydrophobic thermoplastics. How much the improvement of composite mechanical properties could be achieved using this method is still unknown. In this study, ramie fabrics are ethanol-pretreated followed by an atmospheric helium plasma treatment for 15 s, 30 s and 45 s, respectively, in order to reveal the degree of improvement in mechanical properties of ramie-fabric-reinforced polypropylene composites. Scanning electron microscopy shows that the fiber surfaces of the treated groups become rougher and are covered with PP resin after interlaminar shear strength (ILSS) tests. X-ray photoelectron spectroscopy shows that the 30 s treated group has a 50% reduction in atomic ratio of oxygen to carbon. Water contact angle measurement demonstrates that the wettability of the surfaces of the treated fibers significantly decreases. The mechanical tests show increases of up to 39, 28 and 20% in ILSS, flexural strength and tensile strength of the treated composites compared to the control group, respectively, which may be attributed to the combined effects of the increased surface hydrophobicity due to the reaction of ethanol molecules to cellulose in plasma treatments and the roughened surface from plasma etching.