Two feedstocks (LPA and HPA) obtained from modification of naphthenic vacuum residues were selected to prepare mesophase pitch by direct thermal treatment. The influence of reaction temperature, soaking time, reaction pressure, and molecular structures of the feedstock on mesophase develbpment was systematically investigated by analyzing variations in carbonization yield, carbon residue, quinoline insolubles (QT) content, density, optical textures, crystal structure, and surface morphology of derived products. It is found that the mesophase development and the properties of resultant mesophase products were closely related to the molecular structure of the original materials and preparation conditions. Compared With LPA, HPA was the preferable feedstock for thermal treatment beause of its high degree of aromaticity and a large proportion of naphthenic carbon; and the resultant product obtained under the optimum conditions showed large flow domain mesophase, fewer alkyl side chains;. and a high degree of molecular orientation.