Ventilation provides a more comfortable thermal environment for occupants inside of buildings. The growing demand for electricity has encouraged research focused on natural ventilation. In this study, we analyze the thermal comfort in an office room with varied window opening dimensions using the computational fluid dynamics (CFD) technique. We created a 3-dimensional model room with windows on adjacent walls and an atmospheric zone. The flow equations were solved using the control volume method. Steady k-ε turbulence model and incompressible fluid flow of constant property have been considered, neglecting the variation due to temperature. We examined the effects of the window openings’ area and aspect ratio on thermal comfort. The CFD code was compared with the network model and its discrepancy value was less than 8%. Air temperatures along the various midlines of the room were predicted and compared for a range of window sizes. We identified the room areas occupied by the low temperature zone at various planes. Finally, we determined the predicted mean vote (PMV) contours at the midplane to identify the comfort zone. Areas other than the room corners and the areas nearest to the windowless walls are identified as the most comfort zone. Results from this study indicate the optimum window opening area and aspect ratio for maintaining thermal comfort in an office room.