This study proposes a new method to detect the orbital angular momentum of vortex beams using interferometry. Experimentally, when a spatial light modulator is used to generate a vortex beam, coaxial interference directly occurs only by a modulated vortex beam and an incident Gaussian beam which is not fully modulated, producing a perfect petal-like interference pattern. The number of interfering petals coincides with the value of the orbital angular momentum carried by the vortex beam; thus, a separate reference beam is not required when we detect the orbital angular momentum of vortex beams produced by the spatial light modulator. This study analyzes the interference pattern of Bessel-Gaussian and Laguerre-Gaussian beams, which can reflect the orbital angular momentum of the beam, i.e., the azimuth factor and the positive and negative values of the orbital angular momentum together with the radial factor of the Laguerre-Gaussian beam. Notably, the interference results are clear, and the stability is high.