Vortex array in circular synthetic jets is investigated experimentally by instantaneous velocity measurements using a hot-wire anemometer. The ensemble averaged velocities are derived by referencing the oscillatory flow in the orifice. The contours of the ensemble averaged velocity indicate that a vortex ring is generated per a cycle of oscillatory flow. The structures of vortex array are compared for seven strokes of oscillatory flow range from L/d = 0.712 to 7.10. It is found that vortex rings with nearly equal interval along the jet axis travel to downstream and collapse in further downstream. For strokes larger than L/d= 3.6, it is observed the vortex ring accompanies a weaker trailing vortex ring. It is also found that dimensionless convection velocities of vortex ring are independent of stroke. As the stroke is increased, the spatial interval of vortex rings is increased and the location of vortex collapse is moved to downstream that can delay the jet growth in mean jet flow distribution.