The effect of stirring speed and N₂-blowing rate on the mesophase formation during the heat treatment of Naphtha tar pitch at 410-450℃ was investigated. The mesophase content was found to increase with agitator Rey-nolds number and superficial velocity of N₂ gas. However, the mesophase content decreased slightly when the agitator Reynolds number and superficial velocity of N₂ gas were above 24.5, 31.4cm/min respectively. It was also found that the mesophase was deformed by stirring and needle like structures aligned to stirring direc-tion were observed with increasing agitator Reynolds number. The degree of alignment caused by the stirring depend-ed on the stirring speed. The mesophase deformed was rearranged to the spheres by annealing.