Stabilization is a key stage in the production of advanced carbon/carbon composites (ACC) from mesophase pitch, to render the mesophase infusible and the prevention of puffing during the subsequent carbonization. It is generally known that stabilization process as well as properties of mesophase pitch has a great deal of influence on the properties of the resultant ACC. Hence, it is possible to infer the properties of ACC by examing the stabilized mesophase pitch. In this study, extractions by solvents or acidified solvents extraction were carried out from the A-240 petroleum pitch. The extracted pitches were made into mesophase by heat treatments. Oxidative stabilization by air and non-oxidative stabilization by a chemical free radical initiator were performed. When a soluble polymer is fully stabilized, it should become insoluble in solvents. This phenomenon was used to estimated the degree of stabilization. The non-oxygen stabilized mesophase pitch powder was compared with the air stabilized mesophase pitch powder. FTIR provided additional information on the functional groups.