Abstract: Based on the field geological investigation and laboratory geotechnical tests, the authors analyzed the composition, physical properties and engineering geological characteristic of red clay in northwestern Yunnan and then studied their geohazard effects. The composition analysis shows that the grain-size composition of red clay in northwestern Yunnan is mainly the clay-sized fraction. The clay mineral composition is rather complex. Usually the main clay mineral of red clay is kaolinite (K) south of the Lijiang River, while north of the river are illite/ smectite (I/S) mixed-layer minerals dominant. Additionally, the red clay in northwestern Yunnan has the following physical characteristics: high water content, low specific gravity, high plasticity and weak expansibility. The red clay is stable in the natural state, but its strength is greatly decreased with increasing moisture content. Thus geohazards such as landslides and soil erosion easily occur.