Abstract: The droplet transfer of a self-made slag-free self-shielded flux cored wire was studied using high speed photography apparatus. The results showed that the slag spattered continually in the welding process, which can be named as “slag spatter”. The slag spatter occurred not only at the slag-bridge, but also on the surface of droplet, as well as its molten pool. It was discussed that the formation of slag spatter was caused by the great difference of coefficience of linear expansion between slag and droplet, the small volume of slag and the rapid rotation of droplet. The feature of slag spatter was successfully found in explaining where the oxidation products gone, which was also the root cause of weld bond without slag.