Abstract: Relationship between the locating errors and machining errors is studied. By means of geometical analysis, the mathematical model to calculate the deviation of workpiece caused by the location deviation of locators is formulated. So that geometical tolerance can be calculated under the given location errors. Further, optimum method is employed to allocate the locator errors according to the required workpiece geometry tolerance. A simplified method is put forward to facilitate the solution of the optimum process. The method can be used to verify the fixture design. Finally, a case study has been used to support and validate the proposed model.