Abstract: The influence of a nanoSiO2 modified sizing on the tensile strength of carbon fibers was studied by SEM and Xray energy dispersion spectrum tester (EDS). The tensile strength of the resulting carbon fibers was analyzed by the Weibull method. Results showed that when the ratio of SiO2 nanoparticles to epoxy resin was 0.5 w/%，the fiber strength reached a maximum value, 10.1% larger than that of the unsized carbon fibers, and the Weibull parameter exhibited a maximum, indicating a minimum distribution of tensile strength. EDS indicated that the content of elemental Si increased on the surfaces of the sized carbon fibers with nanoSiO2/epoxy emulsion. The sized fibers surfaces were even and the surface grooves were shallow, compared to unsized fibers. However, the fracture crosssections of the sized fibers were concave or convex, indicating a tough fracture character.