Symmetrical folding of a single layer and folding process of an overfold are analyzed by using the finite element method. In the analysis, the constituent substances are assumed to be isotropic and incompressible and to behave as Newtonian fluids. The penalty function method is used to deal with incompressibility, The results of the analysis are compared with the data measured in the field and with those obtained by the elastopla(?)e finite element analysis, It shows that the overfold geometry may be well simulated by using the viscous fluid model. Thus, a possible mechanism of the formation of overfold is proposed.