A new method of measuring the roundness error based on photo electricity is proposed. Roundness error was converted to the displacement of a point on optical axial by this method, to amplify micro-displacement based on ideal optical system longitudinal magnification theory. The displacement fluctuation of light spot image is converted to electric signal outputting by PSD(Position Sensitive Detector).The rotation angle of the piece is measured by circular optical grating when measuring the circularity of the piece, thus we can acquire the information of com- plete cycle outline real time. Assessment of circularity error uses the method of least square app- roximation. We verified the measuring method is correct by opposite experiments. We reach the goals of measuring complete cycle outline, and the accuracy of circularity error is 1μm.