A plasma treating apparatus was newly constructed to modify the surface of carbon fiber (CF) continuously. This apparatus contains a pair of anode rings and a pair of cathode rings. A CF yarn was set along the central axis of the anodes. When a voltage of 400V was applied between the electrodes, plasma was generated around the CF yarn. Oxygen plasma treatment was carried out for two types of PAN-based CF (high strength type and high modulus type). Modification degree of the CF surface was estimated by measuring its wetting force by a dynamic wettability tester. This treatment modified the surface of high strength type CF to be hydrophilic, increasing its wettability remarkably, while no modification was caused on the surface of high modulus type CF.