A sinusoidal phase-modulating two-grating surface profile measuring instrument is reported. This instrument can precisely measure tiny rises and falls of the surface of an object. The (+1,0)th- and (0,+1)th-order beams diffracted from two identical binary step gratings produce a interference pattern with uniform intensity and phase distribution. When one grating is moved relative to the other, the phase of the interference pattern is changed. A sinusoidal phase-modulating technique is applied to measure this phase change precisely. The phase distribution of the interference pattern is independent of the wavelength and intensity of the light source. No change in the measurement results happens when the wavelength and intensity of the light source fluctuate. The instrument is simple in its optical configuration.