This investigation considers the size effect on the deformation behavior of simple tension in microforming and thus proposes a simple model of the tensile flow stress of sheet metal. Experimental results reveal that the measure of the flow stress can be represented as a hyperbolic function , which is a function of (sheet thickness/grain size). The predicted flow stress agrees very well with the published experiment. Notably, a specimen with smaller grains has lower normalized flow stress for a given . Since the material properties of the macroscale specimen do not pertain to the microscale, a critical condition that distinguishes the macroscale from the microscale in the tensile flow stress is subsequently proposed, based on the “affected zone” model, the pile-up theory of dislocations, and the Hall–Petch relation. The distribution of the predicted is similar to the experimental finding that the decreases as the grain size increases. However, the orientation-dependent factor is sensitive to . Hence, further study of the orientation-dependent factor is necessary to obtain a more accurate and, thus, to evaluate and understand better the tensile flow stress of sheet metal in microforming.