Reduction in the dimensional error of an assembly mostly focuses on the variation of fixtures and parts inside, in which the variation is mainly controlled by the methods relating to the fixture design and variation analysis. Recently, more researches concentrate on the dimensional analysis of the aluminum assembly where the distortion of rivet joints cannot be neglected, that is, the riveting-induced dimensional error. Hence, both the device design and the variation analysis for the riveting are drawing much attention to reduce the overall distortion. This article presents a dimensional reduction method, a rivet upsetting direction optimization based on a local-to-global dimensional calculation. The method is developed from a recent framework of assembly process optimization according to the discovered sensitivity between global dimensional error and rivet upsetting directions. A potential function that gathers the overall effect of every locating error is included. Simulations for three riveted assemblies are presented for comparison and results show that the method is efficient for the specific assignment of every rivet upsetting directions, and the effects of locating errors and rivet upsetting directions are highly coupled. The coupled effect yields the key questions in the improvement of this method. Detailed suggestions are then summarized.