Reversal Method (RM) is a well-known method for straightness and roundness measurements. Although this method is very simple in principle, it requires the reversion of the test piece relative to the measurement reference. Such a reversion becomes serious problems in practical application because of operability and accuracy. Authors propose an improved reversal method (IRM) for straightness measurement. In this method, one triangular prism is used as reference piece and is rotated through 180°instead of the test piece. The edge of the triangular prism is measured with one laser sizer, in conjunction with a measurement of surface of test piece by one displacement sensor. The reversion becomes simple and its error can be reduced in this way. In this paper the effectiveness of IRM was discussed theoretically and experimentally as compared with RM. According to the experimental results, both methods are about the same measuring accuracy. Therefore, it became clear that the IRM could be applied for on-machine measurement.