Three dimension modeling is used to design an EXB mass separator. After the perpendicular separating principle and basic properties are analyzed in three dimensions, an E X B model is built and optimized to obtain a homogeneous field with negligible fringing. An example of 30 keV Si+ ions is given to show its mass resolution, ion motion trajectory, and achromatic aberration in both collimated and crossover modes. Results show that the improved separator can operate in both modes in a nanometer optical column, but has better performance using a crossover mode. (c) 2007 American Vacuum Society.