The atrium in the building has many advantages including its aesthetic and functional effect. But the upper part of the atrium has the thermal problem of overheating due to insolation through the window. But natural ventilation aided buoyancy effect can be a solution to make comfortable indoor environment. Proper design of openings is very important to improve thermal environment in the atrium. In this study, thermal evaluations were performed to improve thermal environment in the atrium. Indoor thermal environment of an atrium at Seoul was measured in the field and simulated with Computational Fluid Dynamics( CFD) code. The turbulent flow model adopted is $K-{varepsilon}$ model. The results of computer simulations are compared with the measurements at the point in the atrium. In order to improve the indoor ventilation environment of the atrium, thermal environment evaluations of six alternatives were conducted. After evaluations of the results, the design guidelines of an atrium are suggested.