The size effect on the strength of fiber reinforced plastic（FRP) composite is investigated. The two main factors, statistical size effect and free-edge effe ct, ar e responsible for the size effect in width. Statistical size effect originates fr om material inherent defects and it is described by the traditional Weibull weak es t link theory. Free-edge effect comes into being because of stress singular i ty at edge and ″waste width″ is proposed to link the final failu re strength with free-edge effect. Then a mixed model for the size effect in wi d th is constructed by synthesizing the two effects. The experimental data from D Ku jawski are employed to verify the proposed model and it is shown that the mixed model can predict the final failure strength of specimens with different widths .