The thermal analysis of CubeSats is an essential part of the design of this type ofsatellite, but it is often difficult to achieve due to the need for expensive commer-cial software and the complexity of thermal modelling. In this study, a softwarespecialised on the thermal analysis of CubeSats is developed. This software allowsto easily define a CubeSat model and boundary and initial conditions, and thensimulates its internal thermal behaviour by taking into account thermal conduc-tion, contact conductance, and radiation. The results for a representative CubeSatmodel have been compared to those obtained with Thermal Desktop for an equiv-alent model, and the maximum temperature differences between both programshave been found to stay around 2 to 5◦C.