According to the Weibull theory, the micropore sizes were used to analyze the stress intensity factor of carbon fiber monofilament crack tip stress field. Based on the analysis of carbon fiber monofilament and multifilament tensile strength, diameter and micropore size get the relationship between carbon fiber monofilament tensile strength and the pore radius by the Guinier principle and Griffith fracture theory, thus to estimate carbon fiber fracture toughness. The results show that this method can implement the estimation of fracture toughness on the basis without destroying the structure of the carbon fibers; the fracture toughness of T300 estimated by the average pore size was 1.34 MPa center dot m(1/2), in accordance with data 1.25 and 1.32 MPa center dot m(1/2) by producing defects, errors are 7.2 and 1.5%, respectively.