A sub-1GHz transmitter (TX) integrated chip (IC) with ultra-low power consumption and moderately high adjacent channel power rejection (ACPR) is presented for in-body bio-sensing applications. The 400 MHz 12-phase digital power amplifier (DPA) is implemented with the proposed 16QAM modulation scheme to improve the energy efficiency. The TX IC also contains a 900 MHz FSK TX realized with a symmetrical edge-combiner, which can be used in the low accuracy mode. A fully digital modulator with band shaping is integrated on the chip for the improvement of ACPR performance. Fabricated in 65-nm CMOS process, the chip occupies an active area of 0.75 mm 2 . Under 0.5 V supply voltage, the TX consumes less than 0.66 mW power consumption while delivering ?15 dBm of output power when operating at both bands. The presented TX has an energy efficiency performance comparable to the state-of-the-arts low power designs, with the measured average energy consumption of 64.5/220 pJ/bit, and the measured figure-of-merit (FoM) of 2.04/6.98 nJ/(bit ? mW) for the two bands. Compared with the state-of-the-arts sub-1mW designs in literatures, the ACPR is improved by at least 13 dB.