Raw cotton fiber is water repellent due to the existence of the water repellent cuticle layer. This study is designed to systematically investigate how He/O-2 atmospheric pressure plasma jet (APPJ) treatments influence the wettability and the sizing property of cotton yarns. Water absorption time and adhesion of the sizing agent to the cotton roving are used to evaluate the improvement of wettability and sizing property of the yarn respectively. The water absorption time decreases with the increase of the treatment time and the oxygen flow rate, and the decrease of the jet to substrate distance (JTSD). An optimal water absorption time of 0.8 s is obtained with a treatment time of 20 s, JTSD of 1 mm and O-2 flow rate of 0.2 L/min. Scanning electron microscopy (SEM) shows that the etching effect increases with the decrease of the JTSD and X-ray photoelectron spectroscopy (XPS) presents increased oxygen contents after the plasma treatments. An increase of O-C=O bonds while a decrease of C-OH/C-O-C bonds are observed when the JTSD is set at 2 mm. However, a remarkable increase of both C-OH/C-O-C and O-C=O bonds are achieved when the JTSD is 1 mm. The roving impregnation test results show a nearly doubled adhesion of sizing and a slightly improved breaking elongation, indicating that the plasma treatment does effectively enhance the bonding strength between the fiber and the sizing. (C) 2010 Elsevier B.V. All rights reserved.