A non-contact microwave sensor is presented for in situ process monitoring of nuclear waste glass melts inside a cold crucible induction melting (CCIM) furnace. The level and thermal steady state of the molten glass inside a 1400 °C CCIM furnace were measured during 6-hour long vitrification process using a corrugated horn antenna operating over 20–24 GHz designed for high temperature measurement. The non-contact in situ microwave measurements indicate the ability to measure absolute level, identify thermal steady state of the glass melt, and remotely monitor the vitrification process for safe immobilization of the liquid radioactive waste.