PURPOSE:A petroleum or tar pitch is previously treated at a specific temperature and heated under stirring, while a nonoxidative gan is flown to remove by- products inhibiting mesophase formation at advance and give the titled raw material with high uniformity and good fiber-forming properties. CONSTITUTION:A petroleum pitch obtained as a residual carbon substance resulting from the thermal cracking of vacuum gas oil or naphtha thermal cracking and a coal tar pitch are used as starting materials to effect pretreatment by heating it under a normal or elevated pressure at 300-400 deg.C for 1-24hr, preferably as a nonoxidative or inert gas, preferably a dried gas or nitrogen is passed to prepare an isotropic pitch softening at 60-110 deg.C. Then, the resulting pitch is heated under stirring, as a nonoxidative or inert gas is passed, to obtain the objective raw material without formation of mesophase granules.
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