A stable plasma is generated in argon by using a single needle discharge device. The characteristics of the discharge are investigated by optical methods. Results show that the plasma plume length increases with increasing the peak value of the applied voltage or gas pressure, decreases with increasing the air content. Spatially resolved measurement of light emission is conducted on the plasma plume with photomultiplier tubes. It has been found that the discharge near the needle point has a different behavior with other part of the plasma plume. The discharge in the plasma head originates from a corona discharge which emits light strongly at the negative half cycle, its emission is too weak to be detected in the positive half cycle. The other part of the plasma plume originates from the propagation of luminous cluster (plasma bullet) in the positive half cycle, and every position of which is almost volley in the negative half cycle.