Abstract: In the experiments， with the use of a single needle electrode， atmospheric pressure plasma jets are excited by a repetitive unipolar nanosecond-pulse generator， with working gases such as helium， argon， nitrogen and air. The results show that the jet length increases with the rise of applied voltage； as the flow rate of working gas increases， the length of jet becomes gradually longer and then reduces gradually to saturation after a certain flow rate due to turbulence. In addition， the plasma jets of different working gases have very different appearances. Helium and argon gas jets are needle-like， and the longest jet length is over 7 cm； whereas in nitrogen and in air， the jet is no more than 2 cm long， of brush-like mode.