An apparatus and method for measuring an aperture of a near-field optical probe is provided. The apparatus includes a light source, an optical detector, and a filter. The light source radiates light to the near-field optical probe. The optical detector is positioned before the near-field optical probe and receives the light transmitted through the near-field optical probe to detect light intensity. The filter is disposed between the light source and the optical detector and transmits only light of wavelengths in a specific mode from the light transmitted through the near-field optical probe. Thus, an aperture diameter of the near-field optical probe can accurately be measured in real-time without damaging the near-field optical probe.