Thermal properties and thermal degradation mechanisms at higher temperatures for a homopolymer of 6-vinyl-2,4-diamino-s-triazine (VT) and copolymers with octadecylacrylate (OA) having various compositions were investigated by DSC and TG/FTIR measurements. The melting temperatures of poly (VT-co-OA)’s were higher than that of the OA homopolymer and increased slightly with the VT content. The thermal stability of the homopolymer of VT was rather low and that of poly (VT-co-OA) increased with the OA content, probably because NH2 groups in the VT unit actively reacted with ester groups in the OA unit and others at temperatures higher than 200 degrees C.