One of the traditional problems of the walking robot moving in 3D environment is how to negotiate the boundary of two plain surfaces, which may be convex or concave. In this paper the planning method in the transition region of the boundary is proposed in terms of geometrical view. In each case such as concave and convex, the trajectory that robot moves along has to be determined and the foot positions of the robot associated with the trajectory has to be estimated. The trajectory of the body is derived from the geometrical analysis of the relationship between the robot and the environment. And the position of each foot is estimated by using parameter associated with the hip and the ankle of the robot. The validity of the proposed method is demonstrated through simulations and experiments are performed in the concave and convex environments with limited slope angle.