A new electrode-Tissue impedance measurement circuit integrated with a System-on-Chip (SoC) is proposed and designed for implantable medical devices to monitor the electrode-Tissue contact status. In the proposed circuit, a differential step current is injected into the electrode-Tissue structure to generate a voltage. A differential current switch with a common-mode feedback (CMFB) bias circuit and shaped control signals are designed to generate accurate step currents and stable common-mode bias voltage. Moreover, the measurement circuit shares part of the bio-signal acquisition circuit in the SoC. Thus only an extra power dissipation of 4.3μW is needed at the operating frequency of 15.625Hz. The proposed circuit is fabricated by 0.18 um CMOS technology. The measurement results have shown that the resolution is 17 pp and the maximum error is 3.4% for an impedance of 46k. The proposed circuit is also verified in an in-vitro test by measuring the electrode-Tissue impedance of a subdural grid electrode sunk in the phosphate buffered saline (PBS) buffer.