Abstract: Carbon fiber was considered to be a two-phase composite composed of crystallites and amorphous components. The Mori-Tanaka method was used to investigate the relationship between the microstructure of carbon fibers and the tensile modulus of carbon fibers. In order to follow the dependence of the tensile modulus on microstructure, four types of high modulus carbon fibers M35JB, M40JB, M46JB and M55JB were prepared. The aspect ratio of crystallites was measured by XRD and the degree of graphitization was obtained by Raman spectroscopy. Factors that affect the tensile modulus included the aspect ratio, volume fraction and orientation degree of crystallites. The volume fraction of crystallites was obtained by calculating the micromechanical model. It is found that the higher the degree of graphitization is, the higher the volume fraction of crystallites is. Some relationships are concluded that the tensile modulus increases with the increase of the volume fraction, aspect ratio and orientation degree of crystallites. The three factors are compared. The volume fraction and orientation degree of crystallites have a greater effect on tensile modulus than the aspect ratio of crystallites, and it is only when the orientation degree of crystallites approaches 100%, the effect of the orientation degree of crystallites on tensile modulus may be surpassed by the aspect ratio of crystallites. Comparing the volume fraction of crystallites with the orientation degree of crystallites, it can be found that the orientation degree of crystallites has a greater effect than the volume fraction of crystallites in the initial, but with the increase of the two factors, the effect of the orientation degree of crystallites on tensile modulus is surpassed by the volume fractions of crystallites.