In this investigation, we attempted to accurately evaluate the profile error of the bearing roller convexity, based on the definition of geometric characteristics and shape error of the arc corrected roller convexity line of bearing. This approach involves the least square principle, the method of total least squares fitting, and error evaluation of the convexity contour of the bearing roller. Firstly, the tangent reference points of the arc segment and the straight line are determined by the curvature difference of each measurement point. Secondly, the measurement points on both sides of the two reference points were selected as auxiliary tangent reference points, and a series of least squares arcs were fitted together with the corresponding circular arc measurement points, and the fitting errors were calculated. Then a series of linear equations were determined, and the corresponding straightness error was calculated using the tangent between the straight line and the two segment arcs. The least square fitting and error evaluation of the roller convexity contour of the circular arc modified roller is determined by comparison. The results of the investigated scenario indicate that the total error of the arc modified convex contour curve is 0.020 9, which is 4.5% less than the normal error of 0.02 that is introduced by the standard convex contour curve. This method can effectively evaluate the fitting and error of the convexity contour of the bearing roller and represents a new approach for least squares fitting of planar multi-section curves.