A sinusoidal phase-modulating (SPM) interferometer for real-time surface profile measurement is proposed and analyzed. A laser diode and a high-speed image sensor are used as the light source and the photo detector. The phase corresponding to each measurement point on the surface is calculated by a signal processing circuit. The surface profile is obtained by the phase distribution . The interferometer is insensitive to vibrations of the optical components and fluctuations in the light source. The surface profile of an optical wedge was measured . For 60×60 measurement points, the measurement time was less than 8.2 ms. Repeatability in the measurements was 4.3 nm.