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To enhance the magnetic storage capacity of hard disk drives, the areal density of recording has to be increased continuously. But as the areal density of recording increases to over 100 Gb/in2, the head-disk physical gap, also called flying height (FH), decreases to less than 10 nm. Measurement of FH becomes a significant challenge. In this paper, we propose a symmetrical common-path heterodyne interferometry method to measure FH, utilizing a transverse mode Zeeman laser and a high-speed phase measurement technology, which has the sampling frequency of 100 kHz. We have developed a prototype FH tester based on the method. To effectively compensate influences of the tilt of the rotating disk, vibration of the optical system, and other environmental disturbances on measurement accuracy, we introduced a tri-spot distribution compensation method. Theoretical analysis indicates that the resolution of the FH measurement method reaches 0.1 nm. We carried out a series of experiments of FH measurement with this system, and the results indicate that the measurement system is sensitive enough to FH variation and can measure FH with a high precision and high response. We compared the experimental results with the predictions of FH by numerical simulation of thin-film gas lubrication of sliders, with good agreement.