The separation of water and organic solvents using a double Y-type microreactor was studied. Water and cyclohexane, which possess similar coefficients of viscosity were supplied into a conventional double Y-type microreactor. Mixed liquid was obtained front both outlets, even though the laminar flow was observed under the microscope at the same flow rate. The incomplete separation is considered due to hydrophilicity of the glass channel wall. The separation was drastically improved by use of a microreactor in which plural partition walls are placed at constant intervals in the center of the channel.