In this study, hydrophobically treated stainless-steel microchannel (MC) array chips were used for preparing monodisperse W/O emulsions. A water-saturated decane containing 5 wt% tetraglycerin monolaurate condensed ricinoleic acid ester was used as the continuous phase. A Milli-Q water containing 5 wt% polyethylene glycol (molecular weight 20,000) and 5 wt% NaCl was used as the dispersed phase. The stainless-steel MC array chips used for MC emulsification had a sufficiently high contact angle of the dispersed phase to their surface in the continuous phase. The resultant uniform-sized aqueous droplets with a coefficient of variation of <5% had average diameters of 100-300 mu m, depending on the MC cross-sectional size. The maximum productivity of uniform-sized aqueous droplets reached higher than 1 mL h(-1). The difference in the critical capillary number of the dispersed phase that flows in a 100-mu m depth MC was 1.5 times greater than that in a 30-mu m depth MC.