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One of the limiting factors for the room-temperature continuous-wave (RT-cw) operation of quantum cascade lasers (QCLs) is the high temperature in the active region that stems from the high electrical power and poor heat extraction . In order to simulate the thermal behavior of QCLs, the heat diffusion equation with appropriate source and boundary conditions needs to be solved. However, the heat generation rate of the active region under a given bias is both space- and temperature-dependent. In this paper, we present a method of extracting the heat generation rate by recording the electron-optical phonon scattering during the ensemble Monte Carlo (EMC) simulation of electron transport under different temperatures. The extracted nonlinear heat source together with appropriate thermal conductivity models enable self-consistent calculation of temperature distribution throughout QCLs. We apply the thermal model to investigate the cross-plane temperature distribution of a 9.4 μm infrared GaAs-based QCL . The nonlinear effects stemming from the temperature dependence of thermal conductivity and the heat generation rate are studied.