Among three chip-level electrostatic discharge (ESD) test standards, which were human-body model (HBM), machine model (MM), and charged-device model (CDM), the CDM ESD events became critical due to the larger and faster discharging currents. Besides input/output (I/O) circuits which were connected to I/O pads, core circuits also suffered from CDM ESD events caused by coupled currents between I/O lines and core lines. In this work, the CDM ESD robustness of the core circuits with and without inserting shielding lines were investigated in a 65-nm CMOS process. Verified in a silicon chip, the CDM ESD robustness of the core circuits with shielding lines were degraded. The failure mechanism of the test circuits was also investigated in this work.