The Thermal Building Simulation is a very suitable way for optimizing the thermal performance of buildings. In this paper the thermal behavior of a public building including the heat demand, heat loads as well as the indoor temperatures during summer is simulated using the TRNSYS (Transient System Simulation Program) programpackage. For the start of the thermal building simulation, input parameters concerning the climate data, the building geometry and the use of the building were defined. The influence of the movable shading devices, night ventilation, roof insulation and internal gains were investigated. The results of the simulation showed the possible procedures to arrive at an optimized situation of the building: no window shading devices, shading devices only on the south façade, night ventilation with an air change rate of 6.0[h-1], Roof insulation of 5[cm] and reduction of the electrical power consumption of the PCs to 80[W/PC]. This study will be used as a starting point for a possible refurbishment of this building to reduce the heating loads and to reduce the overheating problem in summer.