Nano-fibers have been applied in many fields because of their unique properties. Quantifying and characterizing the surface free energy and researching the influence of fiber diameters on surface free energies is important. In this paper, three kinds of polyethersulfone nano-fibers with the average diameter of 873, 567 and 254 nm were successfully fabricated by electrospinning at room temperature. The surface dispersive free energy of the three fibers and polyethersulfone (PES) macro-size particles were measured by inverse gas chromatography technique at 30, 40 and 50 degrees C. It was found that the surface dispersive free energy decreased with decreasing fiber diameter. The rationality of the measured values of surface free energy was validated with a BSA adsorption experiment. (C) Koninklijke Brill NV, Leiden, 2012