In this work, the effect of oxy-fluorination on surface characteristics of carbon fibers was investigated in terms of fracture toughness and crack resistance properties of the epoxy matrix composites. The surface properties of the carbon fibers were determined by acid-base values, X-ray photoelectron spectroscopy (XPS), and FT-IR measurements. And their fracture toughness of the composites were evaluated by the interlaminar shear strength (ILSS) and critical stress intensity factor $(K_{lc})$. Crack resistance of the composites was investigated using two types of testing methods, namely critical energy release rate mode I $(G_{lc})$ and mode II $(G_{llc}).;O_{ls}/C_{ls};and;(O_{ls}+F_{ls})/C_{ls}$ ratio from XPS were increased by oxy-fluorination, due to the oxygen containing functional groups and the formation of $C-F_{x}$ bonds on the carbon fiber surface. It was found that the fracture toughness and crack resistance of the composites had been improved by the oxy-fluorination, which could probably be attributed to the increase in the degree of adhesion at interfaces between the carbon fibers and epoxy resin matrix in the present composite system.