Abstract: Differences in the microstructure of carbon fibers are subtle for carbon fibers with similar mechanical properties. The mechanical properties and microstructures of six carbon fibers were investigated by a universal material testing machine, X-ray diffraction, small angle X-ray scattering and Raman spectroscopy to reveal the relationship between the tensile strengths of carbon fibers and their microstructures. Results indicate that the tensile strength increases with decreasing d₀₀₂ or I_D/I_G values, and increases with increasing L_c for five of the six carbon fibers examined. Differences in the tensile strength were characterized by a Weibull modulus increase with increasing micropore radius. The Griffith theory over-estimates the tensile strength of the six carbon fibers. A more accurate formula is proposed to correlate the tensile strength with the microstructures of carbon fibers based on both the Griffith and Weibull theories, which takes into account the tensile strength loss due to the three-layer structure of carbon fibers (an inner-surface layer, an outer-surface layer and a core), defects and the tensile strength of carbon fibers, and is validated by the experimental data on T300 carbon fibers from other researchers.