An Ethylene Bottom Oil Pitch to convert into carbonaceous mesophase was characterized by analytical methods including NMR spectroscopy, elemental analysis, GPC, and sequential extraction by a series of solvents with an increasing dissolving power. Kinetics of the polymerization process during mesophase formation was studied by means of the solubility in quinoline. Results of the experiment showed that the formation of insolubles as the mesophase forms followed the first order kinetics and that the activation energy of the reaction was 50 kcal/mol. Heat-treated samples were examined by using polarized optical microscopy, IR spectroscopy, X-ray diffractometry, and elemental analysis. A systematical removal or addition of a specific composition in the EBO pitch gave significant effect after heat-treatment on the amount of the quinoline insolubles, optical anisotropy, and the X-ray parameters.