A new approach to robot location in an in-house 3-D space using house corners as the standard mark is proposed. A monocular image of a house corner is first taken. Image processing and numerical analysis techniques are then applied to find the equations of the three lines going through the corner point. Under the reasonable assumption that the distance from the camera to the ceiling is known in advance, the position of the robot, on which the camera is mounted, is finally uniquely determined according to 3-D imaging geometry. Experimental results with location error less than 5% on the average prove the feasibility of the proposed approach. Error analysis useful for determining location precision is also included.