Monodisperse O/W emulsification was succesfully performed by metallic microfluidic devices. Microfabrication with metal processing facilitates a high-performance microreactor for chemical process. This is due to high efficiency of heat transfer, industrial strength, chemical proof and facile numbering-up as well as a stable laminar flow. It is well known that periodic production of monodisperse droplets is achieved by colliding two immiscible fluids streams at a T-shaped or Y-shaped junction. In this study, we reported that metallic planar microdevices also brought about the monodisperse O/W emulsion, and which applied to produce nanoemulsion using phase inversion temperature (PIT) method. PIT method enables the emulsification between oil and water with low energy. Since metallic microreactor is operative for heat control, PIT method in the device would save more energy. The size of the resultant emulsion droplets was less than 100 nm with narrow size distribution (coefficient of variance, CV, < 10 %).