Abstract: The algorithm contains the following steps choose a couple of features with symmetry relationship which usually are boundary curves；fit the curves as B-spline or polyline segments,then take the measured part central plane as an initial plane,and project the curves on to it,subsequently calculate the areas surrounded by the projected curves or the distance between them,there exists certainly one plane which makes the areas or distance minimum,the plane is defined as initial symmetry plane；again mirror two contours about the plane,translate the plane to make the distance of points to the contour minimum；finally an approximate symmetry plane can be got.The technique is proved valid by practical applications and has been integrated with the current commercial CAD systems.