Monodisperse de-ionized water droplets 4.5 μm in diameter have been produced in ultrasonic atomization using micro electro-mechanical system (MEMS)-based three-Fourier horn 1 MHz silicon nozzles. The required electrical drive power and voltage are 15 mW and 6.5 V, respectively. The nozzles measure 1.80 x 0.21 x 0.11 cm3 and can accommodate flow rate of 2 to 300 μl/min. As liquid enters the 200 μm x 200 μm central channel of the nozzle, a curved thin liquid film is maintained at the nozzle tip that vibrates longitudinally at the nozzle resonance frequency, resulting in formation of standing capillary waves on the free surface of the liquid film. As the tip vibration amplitude exceeds a threshold (critical or onset amplitude), the standing capillary waves become unstable (temporal instability) and a spray of monodisperse droplets (mist) is produced. The experimental results of resonance frequency, droplet diameter, voltage requirement and critical or onset amplitude support the predictions of the three-dimensional finite element simulation and the linear theory of capillary wave atomization mechanism.