Abstract: In this paper various metal transfer modes in the pulse MIG welding with steel electrode are studied by means of high speed cine camera and oscillograph. Experiments have been made in the downhand and the overhead welding positions. The authors suggested that, though both the monodroplet per pulse and the multidroplet per pulse transfer modes can provide a smooth and strong axial metal transfer, the former exists only in a narrow welding condition region while the latter in a wider region. So the multidroplet per pulse transfer mode is more favourable. There are two critical current levels demarcating the distinctively different metaltransfer mode regions in pulse MIG welding. The first is called the critical pulse transfer current, being the lower threshold current of the monodroplet per pullse transfer; and the second is called the critical arc jumping current, being the lower threshold current of the multidroplet per pulse transfer. The relation between the arc luminous core shapes and the metal transfer modes of the pulse MIG welding are also studied and discussed. The authors have found that the starting of the multidroplet per pulse transfer is always accompanied with an "arc jumping", i. e. the luminous arc root attached to the front surface of the droplet jumps instantly up to the bottom of the constricted neck. This arc jumping phenomenon takes place at a certain amplitude of pulse current with a certain correlated pulse current time. In the conventional MIG welding process this arc jumping phenomenon appears also at the time a globular transfer is converted into a streaming transfer. In essnce the critical current of the conventional MIG welding process is just the critical arc lumping current in a continuous current condition. This is supported by the evidence that by extending the pulse current time the critical arc jumping current in the pulse MIG process will gradually approach to the critical current of streaming transfer in the conventional MIG process. During the process of both monodroplet and multidroplet per pulse transfer the droplet detachment can take place even after the cutoff of the pulse current. This may be attributed to the inertial force of the molten metal in motion and to the instabiility of the thin liquid metal column on the electrode tip.