The off-centre of the camshaft which is mounted on the detector affects the accuracy of measuring the cam outline dimension. The present paper is intended to offer an opposite radial harmonic method that can be used to identify the mounting eccentric magnitude of the measured camshaft. It is proved that the eccentric magnitude identified by opposite radial hamonic methed, is in conformity with the principle of least square. In practical measurement highly-satisfactory results are obtained by employing this method.