A new interferometric system for measuring straightness is developed. In the system, triangular prisms are used to generate two converging beams. A straightness reflector is used to reflect back the converging beams along their respective paths to form a combined beam. The combined beam is received by a detector system to count the interference fringes. Being different from the traditional straightness reflector, the new straightness reflector consists of not only reflecting surfaces but also refractive surfaces. The refractive surfaces cause a change in optical path difference (OPD) when a relative lateral displacement between the triangular prisms and the straightness reflector takes place. The relationship between the OPD and the lateral displacement is derived. The expected results agree with the results in the experiments. A straightness measurement with an accuracy of 0.1 µm is obtained over a travel range of 120 mm.