A proposed large hydropower station dam, with a maximum height more than 300 m, will be built in Southwestern China. It is essential to find out the rock mass structures of the slopes at the dam site. This paper describes grouping and characteristics of rock mass structures based on field survey. The primary of observations are: (1) Structures at the dam site can mainly be divided into three groups: steep dip dykes, steep dip faults and Gentle To Medium Dip Joints (GMDJ). (2) Steep dip dykes and faults have long extension and are weak structures, they may control potential back or side slip surface of the slopes.(3) GMDJ, which nearly parallel to the slope surface, well developed at the dam site. The most unique characteristic of these joints is that they alternately appear in the form of concentrated joints belts, sparse belts (or even no joint belts) and their distribution are closely related to dykes and faults. All structures at the dam site show special characteristics. Further studies will be carried out to explain how they generate and how they affect the slope stability.