When vehicles run on the flooded road, water enters into the engine room and sometimes reaches to the position of air inlet duct and electrical parts and causes the reliability problems. Numerical simulation is effective tool for this phenomenon because it can not only evaluate the water level before experiment but also identify the intrusion routes. Recently, the gap around the engine cooling modules tends to be narrower and the undercover tends to be larger than before in order to enhance the vehicle performance (e.g., aerodynamics, exterior noise). Leakage tightness around the engine room becomes higher and causes to increase the buoyancy force from the water. Therefore the vehicle attitude change is giving a greater impact on the water level. This paper describes the development of water level prediction method while running on the flooded road by using the coupled multi body and fluid dynamics analysis. MPS (Moving Particle Semi-implicit) method was used to analyze the free surface flow and 3-dimensional multi body dynamics analysis was applied to calculate the suspension displacement due to the buoyancy force. Development was conducted in three steps. First is the accuracy validation of MPS method for simple vehicle. Second is the establishment of co-simulation for openings closed full-car to neglect the complexity of the flow in the engine room. Third is the development for full-car as it is. As a result, the accuracy of the water level prediction was within 5% and the time variation was also showed good agreement with experimental results. Additionally, the comparisons between the cases with and without considering vehicle attitude change were conducted and the impact of the vehicle attitude on the water level was clarified. Finally, the mechanisms of the water flow field around the vehicle were revealed and were categorized by vehicle test conditions.