Surface modification of cement reinforcing carbon fiber was tried to improve the compatibility of the fibers with cement. Four kinds of plasmas, oxygen, water, acrylic acid, and diaminocyclohexane, were used to make their surfaces hydrophilic. In all cases, their surfaces became hydrophilic after the treatments. The degree of hydrophilicity depended on the plasma conditions. The highest hydrophilicity was obtained when treateded with acrylic acid plasma. As a result of the modifications, the fibers became more hygroscopic and the degree of cement attachment was improved.