This report deals with the development of a thermal model of the physical package of a micro atomic clock. Several fields of the daily life such as the GPS navigation system or telecommunication now depend on atomic clock. These clocks, based on the energy transition of the cesium atom 133, allow obtaining a very stable and accurate temporal signal. The physical package of an atomic clock is the part of the structure which contains the cesium gas and a laser (VCSEL in the case of an atomic clock). This laser allows questioning the cesium atoms. Nevertheless, the laser and the cesium gas temperatures have an influence on the clock stability.