The importance of controlling microchannel surface wetting by a continuous-phase liquid on stable preparation of oil-in-water (O/W) emulsion was assessed using a flow-focusing stainless steel microreactor. n-Dodecane and 1% sodium dodecyl sulfate (SDS) solution were used respectively as the dispersed phase and continuous phase. The stainless steel microchannel surfaces were modified by mirror and matte finishing, with and without silicon (Si) ion plating. In contrast to mirror finishing, Si plating and matte finishing were more effective for lowering the contact angle of water under ambient air. The combined matte finished/ Si plated surface exhibited the lowest contact angle of water, and therefore the highest contact angle of dodecane in water. For mirror-finished microchannels, O/W emulsion was prepared at the beginning of the run, whereas droplet formation ceased within 30 min, probably because of microchannel surface wetting by dodecane. A more stable preparation of O/W emulsion was achieved for at least 5 h when combined matte finished/ Si plated microchannels were used. Results show that a stable preparation of O/W emulsion depended on complete wetting of microchannels by the continuous-phase liquid and on the low affinity of microchannels for the dispersed-phase liquid in water.