In this work, the ozone treatment of carbon fiber surfaces was carried out to enhance the mechanical interfacial properties of carbon fiber-reinforced epoxy matrix composites. The surface properties of carbon fibers before and after treatments were studied by FT-IR, XPS, and contact angle measurements. Crack resistance of the composites was investigated using two types of testing methods, namely critical stress intensity factor (K$_{IC}$) and critical energy release rate (G$_{IC}$). FT-IR and XPS results showed that the number of oxygen functional groups such as OH, O-C=O, C=O, and C-O, increased on the carbon fiber surfaces after ozone treatment. The crack resistance of the ozone-treated carbon fiber composites also showed higher values than those of untreated carbon fiber composites. This may be attributed to the increase in the oxygen-containing functional groups or the degree of adhesion at interfaces between carbon fibers and the matrix resin in the composite system investigated.ted.