The interfacial adhesion of ultrahigh-modulus polyethylene (UHMPE) fibre-vinylester composites was improved by the oxygen plasma treatment of the UHMPE fibre. The chemical functional group formations on the UHMPE fibre surface by oxygen plasma treatment were analysed using diffuse reflectance Fourier transform infrared spectroscopy and the morphological changes of the UHMPE fibre surface by plasma etching were observed by scanning electron microscopy. The wettability enhancement by the chemical functional group formation and the mechanical intertocking due to the micropits were important factors in improving the interfacial adhesion of the UHMPE fibre-vinylester composites by oxygen plasma treatment. In order to investigate the relative importance of the two factors, wettability enhancement and mechanical interlocking, in the improved interfacial adhesion of the UHMPE fibre-vinylester composites, nitrogen plasma treatment was also performed. Nitrogen plasma treatment of the UHMPE fibre was proved to be effective in the formation of the micropittings and ineffective in the chemical functional group formation in comparison with the oxygen plasma treatment. The interlaminar shear strengths of the nitrogen-plasma-treated UHMPE fibre-vinylester composites showed almost the same value as those of the oxygen-plasma-treated UHMPE fibre-vinylester composites. The wettability enhancement and mechanical interlocking are important in the improvement of interfacial adhesion of UHMPE fibre-vinylester composites by plasma treatment and mechanical interlocking seems to be more important.