The radiation effect on the thermal stability of poly(oxymethylene) diacetate (POMDA) in film has been investigated on irradiation up to 1000 kGy in air with a Co-60 gamma-ray source. The gel permeation chromatography (GPC) indicated that the molecular weight of POMDA was lowered in the polydisperse index (M-w/M-n) approaching the ultimate value of 2 as the radiation dose increased. As observed in the differential scanning calorimetry (DSC), the melting temperature of irradiated POMDA decreased progressively with the lowering of the molecular weight, and the thermally less stable components than the original POMDA were produced in increased yields with increasing radiation dose. According to the thermogravimetry (TG), about 55% of higher-dose irradiated POMDA maintained thermal stability comparable or even superior to that of the original POMDA before irradiation, while about 45% had less stability to thermal degradation. The formation of hydroxyl (G(-OH) = 0.5), formyloxy (G(-OCHO) = 0.6) and methoxy (G(-OCH3) = 0.4) end-groups was identified by the measurement of infrared (IR) dichroism of uniaxially oriented POMDA film after irradiation. The radiation chemical modification of chain-end groups is accounted for by a radical chain reaction mechanism involving random scission of POMDA main-chain as an initiation step. It was concluded that a POM chain possessing at least one hydroxyl end-group is concluded to be thermally less stable than POMDA, while another POM family without bearing any hydroxyl end-group may have thermal stability comparable or superior to the original POMDA.