To manage China’s growing demands of electricity, more and larger dams could be a part of the solution. It is important to evaluate and perform safety analysis for existing dams, in order to improve our understanding and knowledge about future dams. In this master thesis, a safety research of Longtan Dam on the Hongshui River in China was analysed. The main objectives in this research were to construct a 3D model and determine the critical points of large stress, strain and yield area in the dam for different cases.The factor of safety for sliding when primary loads were acting on the dam was calculated to be 0.50 for the non-overflow section of the dam and 0.48 for the overflow section. A safety factor against overturning was also calculated for the non-overflow and overflow section to 2.24 and 1.96 respectively. The results showed that the dam was safe from both sliding and overturning. The numerical results for distribution of stress, strain and yield area were simulated in 3D models by using ANSYS, a finite element program. Several cases for non-overflow and overflow sections of Longtan Dam were analysed for different heights and load combinations. For the first three cases where only self-weight was applied on the dam, high impacts of stress and strain were located at the dam heel, toe and at the largest maintenance tunnel of the dam. Appearance of plastic strain was also found around these areas. However, when primary loads were applied to the dam sections, the values of stress and strain became larger, especially in the toe area. In the last case when the dam was subjected to seismic activity, the distribution of stress and strain along the z-direction (along the dam) showed an uneven distribution, hence showing the importance of simulation in 3D.