This study deals with numerical simulation of air movement and heat transfer in a triangular enclosure for various tilted angles and sizes. The results of the study are given in the forms ofmair flow patterns, temperature contours and pressure contours corresponding to predetermined specific boundarycpnditions. Ventilated triangular enclosure is also included in the analysisfor comparison. The final part of the study is to find the overall heat transfer coefficient. The triangular enclosure is chosen because it can be related to typical roof structure for most builldings in Malaysia. The computersoftware FLUENT/PC has been used to do numerical simulsyion for the desiredphysical models. Computation is done untill convergence so that the results are assured to be accurate. From the final result of overall heat transfer coefficient, it is discovered that a roof installed with ventilator has higher heat resistance than a roof without ventilator. Hence, for the purpose of creating a more comfortable environment in the building, installation of ventilator on the roof can reduce 60% of the heat transfer to the room. It is found that the overall heat transfer coefficient is sssssmaller for bigger tilted angle.