Abstract: The inclined tectonic joints in the loess and their influence on stability of tunnels have been investigated. It is found that there are inclined tectonic joints in the loess, just as in the rock mass. They are weak structural faces in the loess. When a tunnel is dug in the loess, the loess is likely to be cut by these weak structural faces and dug face, forming unstable structural parts which might become the primary factor that can influence the stability of the tunnel. It is suggested that the basic principles and method of rock mechanics should be adopted in designing and construction of tunnels in loess.