There are provided a thermoplastic graft copolymer comprising, as main chain, a polymer having a glass transition temperature of 10.degree. C. or below and, as side chain, an aromatic oligomer having a flow temperature of 100.degree. C. or above; and its preparation process, especially a process wherein an aromatic oligomer containing 50% or more of the structural units represented by the following formula (I): ##STR1## wherein X is O or S; and Ar is ##STR2## wherein R.sup.1 and R.sup.2 are alkyl group of 1-3 carbon atoms or phenyl group, and p and q are an integer of 0-2.The graft copolymer of this invention can be used as a thermal resistant plastic elastomer for heat-resisting hoses, seals, automobile materials, etc.