The concentric circle is taken as target mark. The target parallel to the rudder shaft is fixed on the rudder face. Based on monocular vision, a method for measuring object position and orientation is designed to obtain its angular displacement. Take the diameter parallel to the imaging-plane as a characteristic diameter, and take the diameter perpendicular to the characteristic diameter as an auxiliary characteristic diameter. It is proved that the auxiliary characteristic diameter’s image and two elliptic centers and the projected center are collinear. Based on the invariance of cross ratio, the projected center can be found. Through digital image processing, these characteristic diameter images and their preimages can be obtained. Thus, the relative position and orientation of the concentric circle and its angular displacement can be derived. Simulating test results: measuring range ±85° and RMS error less than 0.04° when Gaussian noise level is 2 pixels. Proposed method has strong ability to resist noise. Measurement errors caused by the target-plane deviating from the rudder-axis direction may be corrected. By this method, the angular displacements of rudder can be calibrated.