Cold atmospheric-pressure plasma plumes have obtained great interests for their attractive features and application potentials. In this paper, a pulsed argon plasma plume was generated in the open air. Characteristic propagation of the argon plasma plume was carried out through high-speed imaging by an ICCD camera. An interesting propagation process was observed for the plasma plume, during which the plasma first propagated in a single streamer channel, and then with side branching as the applied voltage increased. The side branches are generated surrounding the main plasma channel but not split from the anode tip. This branching streamer propagation behavior was much different from the bulletlike development of typical plasma jets or streamers in point-wire or point-plane gaps.