A new approach to the determination of robot locations by common object shapes is proposed. Any object that has a polygon-shaped top and a lateral surface perpendicular to the top can be used for robot location in the approach, as long as the top shape is known in advance. Such objects are seen frequently both in indoor and outdoor environments. In addition, the solution provided by the approach can be computed analytically. These merits make the proposed approach more practical for general applications than other approaches using specially designed marks or requiring iterative computation. From a monocular image of an object, image processing and numerical analysis techniques are applied to extract the projection characteristics of the polygon corners on the object top surface, from which the position and the orientation parameters of a camera-mounted robot can be determined. Experimental results with location errors less than 5% prove the feasibility of the proposed approach. Error analysis that is useful for choosing better viewing angles to get more accurate location results is also included.