This work investigates the temperature rise and heating mechanisms at the facets of quantum well lasers. An analytical solution of the heat conduction equation yields the temperature distribution in the laser and the temperature rise at the laser facets. The heat generation mechanisms are discussed and modeled through a one‐dimensional carrier diffusion equation. The normalized results from the models agree well with available experimental data but the absolute value of the maximum temperature rise is about 5 times lower than that of the measurement. This discrepancy is explained by the reduction of thermal conductivity caused by phonon reflection and transmission at the GaAs/AlGaAs interfaces. Averaging the calculated results over a probe diameter around 1.5 &mgr;m, as is often used in the microprobe Raman spectroscopy measurement of the facet‐temperature rise, reveals that the actual peak temperature at the facet is only 2–5 times higher than the measured value. This is a surprising result considering that the probe diameter is about two orders of magnitude larger than the active region thickness. A detailed examination of the calculation and the existing experimental data suggests a new explanation for the thermal runaway process in quantum well lasers. It is the onset of the absorption by the cladding media, rather than by the active region itself as is commonly believed, that provides the driving force for the thermal runaway process.