The ultra high modulus polyethylene (UHMPE) fiber was treated with argon plasma in order to improve the interfacial adhesion of UHMPE fiber/vinylester composites. Argon plasma treatment of the UHMPE fiber introduces micro-pittings and roughness onto the UHMPE fiber surface. These micro-pittings and surface roughness increase the interfacial adhesion of the UHMPE fiber/vinylester composites through the mechanical interlocking between the UHMPE fiber and vinylester resin. Argon plasma treatment also changes the UHMPE fiber surface into chemically more inert state in comparison with the control UHMPE fiber surface. These chemical changes decrease the efficiency of the mechanical interlocking as the inert surface makes the wetting of the UHMPE fiber by the vinylester resin difficult. In this study, the mechanical interlocking through the micro-pittings is known to play a key role in improving the interfacial adhesion of UHMPE fiber/vinylester composites by the argon plasma treatment. However, the complete wetting of the UHMPE fiber by the vinylester resin is known to be an important requirement for the effective mechanical interlocking between the UHMPE fiber and vinylester resin.