Form-Position error is an important evaluating indicator for piece part. The precision of the evaluating method decides the assessing quality of parts. This paper makes a summary of research on form-position error at home and abroad, and finds the problem on algorithm research. Then according to the new GB, mathematical models of the form error evaluating method is established, and arithmetic with optimization algorithm is designed, based on which, paper establishes the system for evaluating form-position error, and develops evaluation software. The details as follow:1. Based on rules about form and position error in GB/T 1958-2004, mathematical models of form error are established, and arithmetic for all kinds of elements with optimization algorithm including genetic algorithm and golden section method is designed.2. The author proposed a new rotating coordinates convex hull algorithm, and using it in assessing roundness to reduce the computation.3. A system for evaluating form and position error is established, which including part measurement, error separation and data evaluation. And then, the module of Data assessing is built.4. A system of evaluating form – position error is established by using the tool of VC++. The author proposes the function module method to realize algorithm encoding; and operation interface realized with MFC; meanwhile graphics display module adopts Open GL technology. The correctness of arithmetic and useful of software was proved trough a lot of examples in this paper.