The droplet size in a liquid-liquid emulsion can be controlled by placing a metal wire along the centerline of an X-mixer. Droplets gradually form when flowing along the wire, with droplet separation occurring at the tip of the wire rather than at the channel intersection in the X-mixer. The droplet size is now defined by the Plateau-Rayleigh instability developing in the axisymmetric annular flow region rather than by a sophisticated and hardly predictable three-dimensional flow at the channel intersection. The wire-guided droplet formation allows for fine control of the droplet size by changing the wire diameter, the position of the wire tip, and the flow rates. Further control of the droplet size can be achieved by adjusting the surface tension by adding a surfactant.