The effect of the plasma output power on the interfacial adhesion of ultra high modulus polyethylene (UHMPE) fiber/vinylester composites are investigated when the UHMPE fiber is treated with oxygen plasma. As the oxygen plasma output power decreases, the maximum interlaminar shear strength of the UHMPE fiber/vinylester composites increases and the miximum interlaminar shear strength appears at longer plasma treatment time. This phenomenon is thought to be due to effective micro-pitting formations onto the UHMPE fiber surface and mechanical interlocking between the UHMPE fiber and vinylester resin at low plasma output power. At low power oxygen plasma treatment of the UHMPE fiber, the size distribution of the micro-pittings is narrow and the small micro-pittings inefficient to the interfacial adhesion are formed in small number in comparison with the high power plasma treatment. The carbonyl group formation onto the UHMPE fiber surface is favored at low plasma output power and the C=C double bond formation is favored at high plasma output power.