A 30-volt double diffused drain MOS (DDDMOS) is fabricated with standard 0.15μm CMOS process. The substrate current of this DDDMOS is investigated and the two-humps of Ib-Vg curves is observed. The origin of these two humps of substrate current is demonstrated by experiments and TCAD simulation. The cause of first peak is the same as that in conventional MOS device; the second hump is caused by the impact ionization under high electric field in the drift region far away from the channel edge. The correlation between the electric field and the device parameters is studied through the Poisson's Equation and Current continuity equation. Based on the mechanism of the second hump of Ib, its impaction on device reliability is studied.