Abstract: The long term heat stabilized polyalkylene terephthalate resin composition comprises a polyalkylene terephthalate resin selected from the group consisting of polypropylene terephthalate and polybutylene terephthalate, 2 to 60 percent of the total composition of a reinforcing filler and stabilizing amounts of the combination of polyamide polymer, phenoxy resin and an organic phosphonate. The composition retains a higher degree of tensile strength after high temperature long-term aging than does any polyalkylene terephthalate composition of the prior art.