Reactivities of pitches were measured by the sequential solvent extraction method. The formation behavior of bulk mesophase on the reactivity of pitch was observed by hot stage microscope. The formation behaviors of bulk mesophase were depended on fluid flow which was generated by reactivity of pitch. The pitch with high reactivity was formed mosaic like structure of bulk mesophase by coalescence effect of mesophase spherules. Therefore, the structure of mesophase can be predicted by solvent extraction method and mesophase formation behavior when the mesophase is formed by heat treatment from pitch.