We studied thermoplastic poly(phthalazinone ether ketone) (PPEK) resin as a sizing agent on carbon fiber, with emphasis on its thermal stability, surface energy, wetting performance, and interfacial shear strength (IFSS). X-ray photoelectron spectroscopy characterization was carried out to study the chemical structure of sized/unsized carbon fibers. Scanning electron microscopy and atomic force microscopy were used to characterize surface topography. TGA was used to analyze the thermal stability. Meanwhile, contact angle measurement was applied to analyze the compatibility between the carbon fibers and PPEK and the surface energy of carbon fibers. IFSS of carbon fiber/PPEK composite was examined by microbond testing. It is found that carbon fibers uniformly coated with PPEK resin had better thermal stability and compatibility with PPEK resin than the uncoated fiber. The contact angle is 57.01 degrees for sized fibers, corresponding to a surface energy of 49.96 mJ m2, much smaller than that for unsized ones with contact angle value of 97.05 degrees. The value of IFSS for sized fibers is 51.49 MPa, which is higher than the unsized fibers. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013