An ultra-low-power RF envelope-detection radio has been designed for low-power short-range wireless applications. It introduces an interference rejection technique that improves the in-band selectivity by 24.5 dB. The transmitter adopts the power-efficient direct-modulation architecture, and the receiver maintains the simplicity and low power consumption of envelope detection receivers. In the 915 MHz ISM band at 10 kbps the transmitter output level reaches - 3 dBm with 33.9% global efficiency, while the receiver achieves -83 dBm sensitivity with 121 μW power consumption. The Tx and Rx implemented in 90 nm CMOS technology occupy 0.71 and 1.27 mm² , respectively.