This paper reports production of 4.5 μm-diameter monodisperse water droplets using silicon-based one MHz ultrasonic nozzles of a novel design. The novel design of multiple Fourier horns in resonance facilitates pure capillary mechanism atomization. The measured drop diameters in very good agreement with those predicted by the capillary wave atomization mechanism. Due to the resonance effect, the power and voltage requirements for atomization were as low as 15 mW and 6.5 V at atomization rate as high as 300 μ1/min. The droplet diameter was reduced to 4.1 μm when surface tension of the liquid was reduced from 70 dyne/cm (water) to 50 dyne/cm (0.25% Triton X-100 surfactant solution). Such small as diameter drops with GSD (geometrical standard deviation) as small as 1.1 was achieved in ultrasonic atomization for the first time. Note that the fraction of all particles smaller than 5.8 μm in diameter represents the inhaleable fine particle fraction and GSD of 1.3 or smaller is commonly accepted as the standard for monodispersity. Therefore, the MEMS-based MHz ultrasonic nozzle should have very significant impacts on targeted delivery of reproducible doses of medicine to the respiratory system.