With fast development of both hardware and software, 3D real-world scene data, for example multi view stereo mesh, can be acquired efficiently. And since 3D city data are more realistic and more useful in some applications, they begin to appear in the market. Although the generation of 3D city data like multi view stereo mesh can be fast, the data might contain many measurement errors. Because of these measurement errors, some regular objects such as building, ground etc sometimes are not exactly flat. Points which should be on the same plane have small deviations from the plane they belong to, which makes the flat surface bumpy. One way to solve this problem is to control the quality of data acquisition and data processing. Unfortunately, even the data error sources are known, it is not possible to eliminate all the errors, and accurate but expensive data acquisition equipment sometimes are not affordable. In that case, processing existing data is much more economic and time saving compared with collecting data again. This MSc thesis aims at solving above problem. It provides a methodology on straightening planar parts of multi view stereo mesh of the city. After straightening the mesh, this thesis also tries to simply the mesh by removing redundant vertices and faces in the mesh, so that the data will be clean and the data storage can be reduced.