Kinetic studies on the mesophase formation in the pitch were carried out. Naphtha tar pitch(NTP) and straight asphalt(SA) were heat-treated in the temperature range of 410-450℃. The weight loss(V) of the pitches during the reaction was measured and the heat-treated pitches were also fractionated into benzene solubles(R), benzene insolubles/quinoline solubles(S) and quinoline in-solubles(C). The kinetics of the mesophase formation can be analysed by the foolowing multiple reaction model in which the rate equation of each step is assumed to be first order: The activation energies(E) of each step for NTP and SA were 15-65 and 29-61kcal/mole, respec-tively. The kinetic studies indicate that the following heat treatment conditions would be appropriate for preparing a spinnable mesophase: - NTP : a low temperature since E₁<E₂ & E₄ - SA : a higher temperature at an early stage and a lower temperature at a later stage since E₄<E₁<E₂