Characterization of laser diode at high temperature for the issue of long-term reliability to detect and screen the initial failures originated from internal stress and optical instability is performed. Estimation of junction temperature of 1.55 mum Fabry-Perot laser diode was theoretically approached by using thermal resistance of materials regarding the heat path from analytically modeling laser diode. Under the condition of burn-in test, experiments measuring the junction temperature with current injection at controlled temperature verify the result compared to that of analysis.