“N-2-1” principle is widely recognized in the fixture design for deformable sheet metal workpieces, where N, the locators on primary datum, is the key to sheet metal fixture design. However, little research is done on how to determine the positions and the number of N locators. In practice, the N locators are frequently designed from experience, which is often unsatisfactory for achieving the precision requirement in fixture design. A new method to lay out the N locators for measuring fixture of deformable sheet metal workpiece is presented, given the fixed number of N. Finite-element method is used to model and analysis the deformation of different locator layouts. A knowledge based genetic algorithm (KBGA) is applied to identify the optimum locator layout for measuring fixture design. An example of a door outer is used to verify the optimization approach. “N-2-1 ” principle is widely recognized in the fixture design for deformable sheet metal workpieces, where N, the locators on primary datum, is the key to sheet metal fixture design. However, little research is done on how to determine the positions and the number of N locators. In practice, the N locators are frequently designed from experience, which is often unsatisfactory for achieving the precision requirement in fixture design. A new method to achieving the precision requirement in fixture design. metal workpiece is presented, given the fixed number of N. Finite-element method is used to model and analysis the deformation of different locator layouts. A knowledge based genetic algorithm (KBGA) is applied to identify the optimum locator layout for measuring fixture design. An example of a door outer is used to verify the optimization approach.