According to the demand of diameter pipe-line accuracy measurement in the major industry currently, a measure system based on theory of sub-pixel and two CCD-cameras is designed. The system horizontally shoots the steel pipe by two CCD-cameras and determines the location of the steel pipe, cameras, related equipment and the external-inner of measurement system by cameras calibration method. It determines roughly edge areas of the image in two CCD-cameras by the method of differential and determines accurate edge of pipe-line by points in the roughly edge areas in sub-pixel measure method. According to the coordinate attained, external quadrilateral is formed and least squares are used to find the center of the inscribed circle (the center of steel tube) after the diameter is attained. Experimental results indicate that the accuracy of steel pipe diameter is improved, and the accuracy of steel pipe diameter error is below 0.2 percent. According to the research of measurement of steel pipe based on theory of sub-pixel, the system has simple structure, strong resistance to vibration performance and high accuracy, which is applied to industrial high-speed on line detection.