The conditions of generating a single plasma channel in air are studied by using picosecond and femtosecond laser pulse. A stable and single plasma channel can be generated by a picosecond laser pulse with energy of 8—12mJ and focusing length of 15cm. The self-generated light emission is observed to be confined in the plasma channel and propagates in the channel. When using short focal length lenses it is easy to generate a single plasma channel in air with femtosecond laser with energy less than 10mJ.