A combined model of surface dissolution of the drug and bulk erosion of the polymer is developed for describing the biphasic release profile of drugs from biodegradable polymer rod devices. The model parameters are determined from the conventional moment analysis of the time course of the release profile. The model describes well the typical biphasic phenomena of drug release, the initial bursting release due to dissolution of the surface drug molecules, and the late bursting caused by the bulk erosion of biodegradable polymers.