We present a technique for the continuous interphase synthesis of metal oxide nanoparticles using a droplet flow reactor. Conducting the synthesis reaction inside droplets of controlled volume offers several advantages, such as eliminating temperature and concentration gradients inside the reactor as well as preventing reactor fouling. The synthesis reagents are initially located in separate phases, and reagent addition is accomplished through diffusion from the bulk phase to the droplet phase. In this work, the technique is demonstrated by synthesizing zinc oxide nanoparticles inside aqueous droplets containing zinc acetate in a bulk stream of sodium hydroxide in 1-octanol. This flow synthesis of zinc oxide nanoparticles provides more control of the nanoparticle morphology and has a narrow particle size distribution as compared to that of a batch reactor. The size and morphology of the nanoparticles are shown to be easily controlled from 41 to 62 nm by varying zinc acetate and sodium hydroxide concentration, reaction temperature, and residence time. Spherical as well as platelike shapes of zinc oxide nanoparticles are seen for the ranges of parameters studied. This technique can be used to synthesize a variety of metal oxide nanoparticles of controllable size.