Characteristics of tool wear are theoretically studied. Theoretical results agreed with experiment. The tool wear in cutting of GFRP is abrasive. As glass fiber has the flexural rigidity and is held in visco-elastic material, the contact pressure between glass fiber and tool changes with fiber orientation and cutting conditions. This causes the peculiar tool wear characteristics. (i) Dependence of the rate of tool wear on cutting speed : With increase of cutting speed, the rate of tool wear initially increases gradually (region I), then it increases proportionally to cutting speed (region II), and finally the rate is constant beyond a critical speed (region III). (ii) Dependence of the tool wear on chip thickness before cutting (t) is either one of following two cases (a) and (b) or intermediate between (a) and (b). (a) : The tool wear rate decreases in the region I and does not change in the regions II and III as “t” increases. (b) : The tool wear rate increases in the regions I and II, and does not change in the region III as “t” increases. The intermediate situation between (a) and (b) is the case where the tool wear rate decreases in the region I, and increases in the region II and does not change in the region III as “t” increases. (iii) The rate of tool wear decreases with softer matrix. This theory can be applied for cutting not only of GFRP but also of the composite materials composed of hard fiber and plastics such as CFRP.