The turn-on mechanism of SCR device is essentially a current triggering event. While a current is applied to the base or substrate of a SCR device, it can be quickly triggered on into its latching state. In this paper, the complementary substrate-triggered SCR devices, which are combined with the substrate-triggered technique and SCR devices, are first reported in the literature for using in the on-chip ESD protection circuits. A complementary style on the substrate-triggered SCR devices is designed to discharge both of the positive and negative ESD stresses on the pad. The total holding voltage of the substrate-triggered SCR device can be increased by adding the stacked diode string to avoid the transient-induced latchup issue in the ESD protection circuits. The on-chip ESD protection circuits designed with the proposed complementary substrate-triggered SCR devices for the I/O pad and power pad have been successfully verified in a 0.25-μm STI CMOS process with the HBM (MM) ESD level of >8kV (650V) in a small layout area.