Responding to the market demand for high quality and production limited quantity and variety, the automation technology of flexible manufacturing system or computer integration manufacture sytem has being developed. During the automation manufacturing, to some extent the instability of product quality would be resulted from different precision of machine tool, qualification of operators and manufacturing technology. In addition, the utilization of traditional mannual measurement or off-line contact quality inspection would reduce productivity. Therefore, the technology of non-contact on-line product quality inspection is becoming a very important research issue. This report is focusing on the setup of non-contact automatic measurement of concentricity by applying personal computer and image processing technology. LOG and zero-crossing method is used to complete image edge detection. Then curve-fitting is applied to generate the inner circle and outter circle of mechanical part. Subsequently, the coordinate of edge points of the inner and outter circles, together with the least-square error method, will be applied to estimate the concentricity of the mechanical part waiting for inspection. Comparing the result of experiment with the data measured by a FEDERAL high-precision roundest equipment the proposed system is proved with good repetitivity and accuracy.