Rheological characteristics of two pretreated petroleum pitches, during their transformation to mesophase, have been studied by using rotational viscometer. Simultaneously, the formation, growth, coalescence of mesophase spheres, and the alignment of coalesced mesophase were observed by Hot-Stage-Cinema-Microscope, discussd with the results of viscosity change and the Q.I. content. It is found that both pretreated pitches were Newtonian fluids at low temperature, but behaved pseudoplastics as increase of mesophase content above 390C. Shear thinning behaviour was also apparent in the shear rate range of 1.9-20 sec^-1 at temperature above 390C. The apparent viscosity-temperature curves of this pyrolysed pitches were much affected by the pretreatment conditions.