The springback behavior of thin metal sheets will significantly affect the forming accuracy of microchannels during microstamping process, especially as the forming scale reduced to micro/meso levels due to the size effects and grain structures. This paper aims to study the size effects on springback behavior and microchannel’s formability in microstamping process. The thin sheet of SUS304 specimens with different grain sizes were prepared and employed in uniaxial tensile tests to examine the stress-strain relationship and used to study the effects of grain sizes on the material’s plastic deformation behavior. A constitutive model incorporate surface layer and grain structures is developed to characterize the deformation behavior of thin-metal sheet with different grain sizes. Then, the constitutive model is utilized in finite element simulation of microstamping process to fabricate microchannel structures, and the results are verified by experimental testing. Both numerical simulation and experimental results showed that the size effect is critical in microstamping process, the developed constitutive model is able to accurately characterize the global springback behavior of thin metal sheets during microstamping process.