Abstract: This study systematically investigates the degradation mechanisms of silicone encapsulants in high-power LED packages under thermal stress. The silicone material was characterized before and after thermal aging using scanning electron microscopy (SEM) for morphology observation, energy-dispersive X-ray spectroscopy (EDS) for elemental analysis, thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC). The results indicate that thermo-oxidatively aged encapsulants exhibited brittleness, cracking, and other failure phenomena. Compared with unaged silicone, the aged specimen displayed a significantly rougher surface with suspected particulates. EDS analysis revealed substantially higher silicon content in aged/cracked regions than in intact areas. FTIR characterization further confirmed pronounced oxidative degradation of side chains in failed specimens. Thermal degradation and phase transition profiles derived from TGA and DSC suggested that silicone undergoes simultaneous degradation and cross-linking under thermo-oxidative stress. Based on comprehensive characterization and observations, the primary failure mechanisms are dominated by two pathways: oxidative cross-linking of side groups and cyclization depolymerization of the main Si-O chain.