Based on the theory of dimension chain,the dimension chain compute with extremum method and probability method is researched. It defines that compute method determined by the number of composition loops,the real statistical data and the statistical process control ability can be used to optimize the dimension chain. Usually,the structure of gearbox is complicated,but the accumulative error must be controlled in a minor range. That method can avoid severe dimension tolerances of components and parts in the design of gearbox,meanwhile products can be manufactured reliably and economically.