Abstract: As to the existence of water in atmospheric pressure plasma (APP) treatment, the influence of environmental moisture on the effects of atmospheric pressure plasma treatment of hydrophobic fiber was investigated. Ultrahigh modulus polyethylene fibers (UHMPE) fibers were treated using atmospheric pressure plasma in three different relative humidity levels, namely 5, 65% and 95%. The static contact angle and the interfacial shear strength (IFSS) of the control and the treated fibers were measured. The morphological and chemical changes on the fiber surface were characterized by Atomic Force Microscopy (AFM) and X-ray Photoelectron Spectra (XPS) respectively. After the plasma treatments, the surface roughness, the number of oxygen containing groups and IFSS increased while the water contact angle decreased and the tenacity din not change. The contact angle and IFSS did not obviously decrease and increase as the environmental moisture content increased. It was demonstrated that the moisture in the APP environment did not have a significant influence on effects of atmospheric pressure plasma treatment of hydrophobic fiber.