A process for co-production of a pitch for the manufacture of HP carbon fibers and a pitch for the manufacture of GP carbon fibers is proposed. The pitch for the manufacture of GP carbon fibers is prepared from the spent fraction not used in the production of an optically anisotropic pitch suitable for the manufacture of HP carbon fibers. The spent fraction has hitherto been discarded as a valueless material. According to the present process, a pitch for the manufacture of so-called ultra HP carbon fibers with tensile strength of over 400 Kg/mm.sup.2 and modulus of elasticity of over 60 ton/mm.sup.2 and a pitch for the manufacture of GP carbon fibers can be produced simultaneously. Both pitches have very excellent spinnability and when they are spun, they cause no fiber cut-off even at a high spinning rate of, for example, 500 m/min or 700 m/min. One important merit of the process is that the production ratio of ultra HP carbon fibers and GP carbon fibers can easily be changed to accommodate to market's demand. Accordingly, the process has wide flexibility in operation. Of course, the effective utilization of valueless spent fraction can reduce the production costs of not only ultra HP carbon fibers but also GP carbon fibers.