Formation of quinoline insoluble and mesophase obtained from coal tar pitch by heat treatment in the temperature range from 410℃ to 450℃ for various times have been studied, using solvent extraction, polarized microscope and SEM. Content of quinoline insoluble and mesophase were not equal to each other, contrary to the facts known by this time. At an early stage of heat treatment, the rate of formation of quinoline insoluble was greater than that of me-sophase, and there were some isotropic secondary quinoline insoluble. At a later stage, the rate of formation of me-sophase was greater than that of quinoline insoluble. The rates of formation of quinoline insoluble and mesophase were well fitted with 1st order reaction and activation energies of those were 36.6Kcal/mole and 58.5Kcal/mole, re-spectively. Content of volatile matter and C/H ratio were changed remarkably during nucleation of mesophase, but changed lit-tle during coalescence of mesophase.