Abstract: The transformation behavior of mesophase derived from three coal tar pitches with different softening-points was investigated. FTIR and XRD were used to analyze the compositions and molecular structures of pitches. The transformation behavior of the pitch to mesophase was observed in situ by a polarizing light microscope equipped with a heating stage. A correlation of the composition and molecular structure of the pitches to the development of the mesophase was obtained. Results indicate that the thermal conversion behavior of the three pitches to mesophase were different, and this was closely related to their aromaticity and quinolone insoluble fraction. The nucleation rate of mesocarbon microbeads was higher than the growth rate for the coal tar pitch with a high content of quinolone insolubles. A molecular lamellar structure with a high aromaticity benefitted the growth of microspheres and the formation of mesophase.