Theoretical considerations and experimental results related to the principle of dual-wavelength absolute distance interferometry (ADI) are presented. It is predicted that the required initial uncertainty of a length to be measured was related to the synthetic wavelength, its stability, and the length to be measured. An absolute distance measurement approach employing a dual-longitudinal-mode He-Ne laser is proposed. In this approach, a heterodyne interferometer with two acousto-optic modulators (AOMs) is used to directly measure the fractional fringe at the synthetic wavelength formed by them. The common optical path configuration in the simplified interferometer eliminates the influences that air turbulence and mechanical vibration have on measurement. Factors that influence measurement accuracy are analyzed theoretically and experimentally. Mode mixing is determined as the main influence on the accuracy of the approach, and the error caused by it can be compensated through data processing if the amount of mode mixing is fixed. Additionally, an experimental result is presented that obtains an accuracy of 140 μm for 25 m. © 1999 Society of Photo-Optical Instrumentation Engineers.