Arcs in DC relays have the characteristics of short burning time and little brightness. A high speed photograph experiment was designed and performed in this paper. In the experiment, the whole burning processes of arcs were captured and some parameters such as arc diameter, length and grey level were obtained from the photos by digital image processing technology. Combining with the waveforms of arc voltages and currents, the relationships of arc voltages & arc lengths, arc currents & arc diameters by fitting functions were obtained as well. On the other side, the movement speed curves of contacts and some analysis on contact bounce were discussed in the paper. The results may be referred to when manufacturers design the parameters of small power DC relay.