We report a new MEMS device for continuous separation of particles based on size. Unlike previous passive fluidic devices for particle separation, fluidic streamline design based on local geometry and fluidic resistance of side channels are used. The filtering effect is achieved not by physical device boundaries, but by the collection zones established by fluidic field. The separation region of the device has a small footprint of 1.5mm by 0.8mm for separation of particles in micron range. The particles are automatically collected in different exit channels after they are separated, which facilitate further sensing and processing. Like crossflow filters, particles are separated perpendicular to flow direction. The minimal feature size of the device is designed to be larger than the diameter of the largest particles, so clogging can be minimized. Solvent exchange can be accomplished for larger particles. We demonstrate separation of 5m and 10m polystyrene beads, as well as erythrocytes and leukocytes of human blood with better than 96% efficiency.