This paper describes an atomic force microscope (AFM) based instrument for 3D edge profile measurement of single-point diamond cutting tools. The instrument is composed of an AFM unit and an optical sensor for alignment of the AFM probe tip (silicon cantilever) with the diamond cutting tool edge. In the optical sensor, a laser beam from a laser diode along the Y-axis is focused to generate a small beam spot with a micrometer-order diameter at the beam waist, and then received by a photo-detector (photodiode). The tool edge top and the AFM probe tip are brought to the center of the beam waist in the XZ-plane through monitoring the variation of the photodiode output, respectively. Consequently, the AFM tip can be aligned with the tool edge top. Alignment experiments and 3D edge profile measurements of a round-nose type single-point diamond tool are carried out.