Many mechanical and optical components contain step features whose surfaceheight changes far exceed the optical wavelength. Therefore, this work presents aninterferometer based on variable synthetic wavelength interferometry (VSWI) anddifferential heterodyne configuration to measure large step heights directly andunambiguously. This largely common-path configuration can substantially reducethe influence of environmental disturbances, which are the main sources of error inthe VSWI. Only one external cavity diode laser (ECDL) is employed to synthesizea series of synthetic wavelengths in descending order. The wavelengthsare combinations of the varied wavelengths and the initial wavelengthof the ECDL. In contrast to wavelength scanning interferometry, thismethod does not require the laser wavelength to be continuously tuned.The step height is sequentially measured at these synthetic wavelengthsand a lock-in amplifier resolves the corresponding synthetic fractionalfringes. The step height is determined following a succession of opticalpath difference calculations, in terms of the synthetic wavelengths andmeasured synthetic fractional fringes. Three known step heights, verifiedby a gauge block interferometer, were used to confirm the performanceof the proposed system. The results reveal that the uncertainty in themeasurement is approximately 80 nm when the measured height is up to 25 mm.