The interference pattern between two freely expanding Bose-condensed gas with quantum vortex was studied and compared with the situation without vortex. The results show the interference fringes in vortex state tilt and merge at the location of vortex core. So the direction of vortex can be judged by the tilt direction of interference fringe, and the vortex quantum of circulation can be established by the number of merged fringes at the location of vortex core. This special interference pattern offers effective method in proving the existence of vortex experimentally.