The far-field diffraction pattern of a vortex beam by a rhombus aperture is investigated theoretically and experimentally. It is found that the number of the dark stripes in the Fraunhofer diffraction intensity distribution is just equal to the topological charge value of the measured optical vortex, and the centre of each dark stripe is just a phase singularity point. Based on this property, it provides us a simple way to detect the orbital angular momentum (OAM) of an optical vortex beam.