Abstract: This article reported an investigation on the chemical and physical evolutions during the thermal degradation process of aluminum diethylphosphinate (AlPi). The experimental results indicated that the phosphorus species were released to the gas phase through the sublimation of AlPi. On the other hand, the competitive thermal decomposition might reduce the flame-retarding efficiency by leaving phosphorus species in the solid phase. At the temperature close to a flaming temperature, the sublimated AlPi was further split into phosphorus atom, which could form the flame-retarding radical scavengers accordingly.