In developing unit operations for microreactor systems, an Archimedean microchannel was fabricated and used to separate solid particles (glass beads of 22 and 40 pm in diameter) from water. The spiral channel was 700 mu m in width and 500 pm in height, with an innermost radius of 2.5 mm, and an outermost radius of 5.0 mm, and was held either horizontally or vertically. After 2.5 turns, the channel was bifurcated to the outlets. The slurry was introduced into the inner end of the microspiral. Solid particles moved along with the secondary flow (Dean vortex), and, at the same time, ascended to the lower walls due to gravity. The samples recovered at the outlets were dried and weighed, and the separation factor of particles was determined. Horizontal orientation: When the sedimentation velocity of the particles was higher than the central velocity of the secondary flow, particles were concentrated in the inner bottom corner of the channel. Vertical orientation: In this study, the outlets were located at an angle of 90 degrees relative to the top of the channel. Concentration of solid particles varied along the direction of the liquid flow. Since the bifurcated outlets were located in the downward liquid flow region, solid particles were concentrated on the inner wall of the channel. CFD simulation was also performed. The calculation was approximately in agreement with the experiment. Calculated distributions of liquid and particle velocities, as well as those of particle concentration in the microchannel, gave beneficial suggestions for the design and operation of microspirals applicable to liquid/solid separation.