During the deformation process of micro-bending, the size effect resulting from the variation in the grain size of the microtubes is the primary factor affecting the bending behavior. The current study on free bending mainly focuses on using macroscopic finite element simulation and experimental analysis to investigate the factors affecting the bending radius and forming quality. However, due to the size effect, the practices and insights gained from macroscopic forming may not directly apply to microforming. Thus, this study aims to reveal and explain the reason for the increase in the bending radius of microtubes with decreasing grain size by combining experimental investigations with crystal plasticity finite element modeling (CPFEM). It was determined that the grain size is a crucial consideration in the micro-bending process of microtubes. Additionally, the CPFEM approach can provide guidance for the process parameters of free bending forming of microtubes.