An ultra-lower power (ULP) MEMS-based FSK transmitter is implemented in 65 nm CMOS. The transmitter operates in ISM-915 MHz band using a binary FSK modulator employing the high-Q dual microelectromechanical system (MEMS) resonators. The transmitter using an adaptive fast switching binary FSK modulator generates an FSK signal in 915 MHz band with -8.2 dBm output power, supporting up to 10 Mb/s data rate. The phase noise (PN) of the FSK modulator was measured for each of the two different operation frequencies, achieving -139.4dBc/Hz and -138.7dBc/Hz at a 1-MHz offset at 911.9 MHz and 923.1 MHz, respectively. At 10 Mb/s, the transmitter consumes 753 µW, which translates to energy efficiency of 75.3 pJ/b.