A fully integrated low dropout regulator (LDO) with decoupling capacitors (decaps) for high electromagnetic immunity is designed in the standard 0.18 μm CMOS technology. The decaps composed of MOS and MoM capacitors are utilized to decouple the high frequency interference. The characteristic of electromagnetic susceptibility (EMS) of the LDO is performed by the direct RF power injection (DPI) measurement up to 18 GHz. The measured results demonstrate the immunity of LDO with decaps is superior to that of LDO without decaps (maximum improvement of 11.6 dB). Also, the wideband DPI measurement is shown to be capable of characterizing the EMS of ICs.