In this paper, the generation mechanism of jet noise containing discrete frequencies with peaks at a specific frequency was studied by the digital image processing. The flow of subsonic jet issuing from a nozzle with exit divergent angle 20 degrees was photographed by a high-speed video camera included Schlieren device. The phase speed and distance of vortex rings were calculated by the digital image processing. It was clarified that the fundamental frequency of the noise power spectrum was closely related with the separation vortex, and agreed well with the frequencies of vortex rings released from the nozzle.