We proposed the balanced vertical double - diffused MOS (B-VDMOS) transistor. The B-VDMOS transistor is not destroyed by avalanche breakdown and acquires the high second breakdown current. Owing to the high second breakdown current, the B-VDMOS transistor has high electrostatic discharge (ESD) robustness. This paper presents the mechanism of the snapback phenomena and clarifies the cause that the B-VDMOS transistor has the high second breakdown current. We find the cause that current does not become concentrated even after avalanche breakdown in the B-VDMOS transistor. Copyright © 2009 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.