This paper presents a WBSN SoC with a 0.9V/165μW MCU and dual-band RF. The SoC contains a 403MHz transceiver and a 915MHz receiver, of which the 403MHz band is composed of a 200kbps FSK transmitter and a 64kbps OOK receiver, consuming 5.58mW and 3.13mW, respectively. The 915MHz receiver based on energy harvesting gives the SoC the unique high-efficiency work-on-demand capability, avoiding wasting energy during the idle-listening period. The power consumption of the integrated MCU is only 1/3 of the previous SoC designs in WBSN applications, implementing the on-demand MAC protocol with instant response, satisfying the requirements of medical wireless body sensor network (WBSN) applications.