We have developed an optical technique forinsitumeasurement of temporal change and spacial profile of Si lattice temperature during cw laser annealing. This technique utilizes time‐dependent optical interference in Si films on insulator. By using a microscope for focusing a laser probe beam to 2.0 &mgr;m, time‐resolved and space‐resolved Si lattice‐temperatures were measured below the melting point (1412 °C) on a time scale of 10−4–100s during cw laser annealing of Si on sapphire.