This paper focuses on thermal response in rear part of engine on HIMICO (High Mach Integrated Control experiment). Firstly, pyrolysis reaction of CFRP, which is used in HIMICO as thermal protection system, is modeled from previous experimental data. Comparison between numerical data and experiment data shows good agreement. Secondly, one-dimensional heat transfer analysis with obtained pyrolysis reaction model on the external nozzle is conducted. By comparing with ramjet engine experiment, calculation result is confirmed to be appropriate. Thirdly, in addition to the thermal response analysis, CFD analysis in the rear part of the engine is conducted. Calculation result indicates that ramjet engine combustion test overestimates heat load on external nozzle compared to actual flight condition because of high back pressure.