We present a new digital phase measuring scheme for opticalheterodyne interferometry, which provides high measurable velocity up to6 m s-1 with a fine displacement resolution of 0.1 nm. Themain idea is combining two distinctive digital phase measuring techniques withmutually complementary characteristics; one is counting the Doppler shiftfrequency with a 20 MHz beat frequency for high velocity measurement and theother is synchronous phase demodulation with a 2.0 kHz beat frequency forextremely fine displacement resolution. The two techniques are operated inswitching mode in accordance with the object speed in a synchronized way.Experimental results prove that the proposed dual mode phase measuring schemeis realized with a set of relatively simple electronic circuits for beatfrequency shifting, heterodyne phase detection and low-passfiltering.