In this paper, we present ‘Tuskbot’, a wheel-based robot with a novel structure called ‘Tusk’, passive and protruded elements in the front part of the robot, which can create an angle-of-attack when it climbs stairs. The robot can easily overcome stairs with the help of Tusk, which does not require additional active mechanisms. We formulated a simplified mathematical model of the structure based on the geometrical relationship between the wheels and the stairs in each phase during the stair-climb. To test the model and Tusk structure, we calculated the length of each link and the angle of Tusk from the dimension of stair and radius of wheels, and built the robot accordingly. The results demonstrate the validity of the model and the structure.