Recently, there has been an increase in emphasis on small satellites, however their small size results in small surface areas which often imply thermal and power constraints. For this reason, thermal analysis and design have become of crucial importance for every phases of a space mission project. In particular, since satellites limited power, the active thermal control systems are less employed, so it is important properly choose the passive systems and exploit them to the full. The aim of this study is to present a quite complete description of a thermal analysis, starting from discussing the thermal problem in every aspect, to describing the mathematical methods, which are at basis of the analysis, and finally presenting the computer thermal model.