Abstract: The Giseeler fluidity method was used to investigate the rheological properties of pitch with and without heat treatment from room temperature to 520 ℃. Results show that the fluidity has a good repeatability at different temperatures under different heating rates. The fluidity of the raw pitch slowly increases from 253 to 282 ℃, then increases exponentially from 282 to 311 ℃ near the softening temperature, and finally remains unchanged from 311 to 498 ℃. This changing pattern of fluidity with temperature has also been found for heat-treated mesophase pitch and can be formulated by the Arrhenius equation, from which the viscous flow activation energy can be obtained. The toluene-insoluble fraction increases from 67.3% to 88.7% with heat-treatment time and the flow activation energy of the treated mesophase pitch in the Newtonian fluid region at low temperature range is between 203.6 and 294 kJ/mol.