We have determined the junction temperatures of laser diodes mounted on ceramic submounts and hermetic transistor-outline (TO) packages using data from wavelength-shift measurement as well as finite-element modeling (FEM). We found that the difference in the junction temperature between chip/submount and chip/submount/TO measured at the free-standing room-temperature condition cannot be used as the reference data for the design of accelerated life-test condition at hot oven. For the free-standing condition, the heat spread from the active region of the chip to the chip substrate, submount, and TO. Hence, there was an additional thermal gradient in the chip/submount/TO arising from the thermal impedance of the TO header. For the oven condition, the heat dissipation via forced convection from the outer surface of the TO was important. Hence, the thermal gradient of the TO resulting from heat conduction became smaller. Using the FEM, we showed that the simulated junction temperature was in close agreement with the experimental value. The heating difference between the chip/submount and chip/submount/TO was smaller at the oven condition than at the free-standing condition.