The effect of anodic surface treatment on the polyacrylonitrile (PAN)-based carbon fibers surface properties and the mechanical behavior of the resulting carbon fiber-polymer composites has been studied in terms of the contact angle measurements of fibers and the fracture toughness of composites. Results from contact angle measurements revealed that the angle of electrolyte solution largely decreases with increasing current densities of treatments up to 0.4-0.5 A m-2. The results obtained from the evolution of KIC with flexure of the composites as a function of electric current density shown that the KIC of the composite continually increases with increased current densities of the treatments up to 0.5 A m-2, and a maximum strength value is found about 294 MPa cm1/2 at the anodic treatment of 0.5 A m-2. It can be concluded that the anodic surface treatment is largely influenced in the fiber surface nature and the mechanical interfacial properties between the carbon fiber and epoxy resin matrix of the resulting composites, i.e., the fracture toughness. We suggest that good wetting plays an important role in improving the degree of adhesion at interfaces between fibers and matrices of the resulting composites.