Abstract: Polyacrylonitrile (PAN)-based carbon fibers (CF) with nano-SiO₂ modified emulsion sizing and unmo-dified emulsion sizing were prepared. The stability of both emulsions was studied by the static precipitation method and optical microscopy. Scanning electron microscope (SEM), X-ray energy spectrum tester (EDS), atomic force microscopy (AFM) and dynamic contact angle analysis (DCAA) were used to study the surface property changes of the CF without sizing and the CF with unmodified sizing and modified sizing. The interfacial adhesion properties between CF and the matrix resin were evaluated by the single fiber fragmentation test. The results show that the size of both unmodified and modified emulsion particles is little and both emulsion stabilities are better; moreover, the former is more excellent than the latter. The surface energy and toughness of CF are improved after sizing. Furthermore, the surface energy and toughness of the CF with modified sizing have maximum values. The interfacial shear strength (IFSS) value of the CF with modified sizing is 27.2% higher than that of the CF with unmodified sizing. The harmonic mean work of adhesion (W^(h)_a ) and geometrical mean work of adhesion (W^(g)_a ) between sizing modified carbon fibers and the matrix resin are also increased by 12.7% and 11.7%, respectively, compared to those of the CF with unmodified sizing.