<p>This research was aimed at exploring the effect of grain size on the mechanical behavior and deformation mechanisms of pure Ti thin plates. The 0.1&#xa0;mm-thick pure Ti thin plates of different grain size were used in a mesoscopic uniaxial tensile experiment. The mesoscopic tensile experiments were performed with a very small measurement error (0.2&#xa0;N). It was verified that the true stress increased during the plastic deformation, and the material elongation decreased as the grain size increased. The fracture mechanisms of the material gradually changed from ductile fracture to a mixture of ductile and brittle fracture, and then became brittle fracture. By analyzing the microstructural evolution, it was found that the frequency of deformation twins and the deformation coordination ability of grains decreased. The promotion of twinning on deformation was weakened with increasing the grain size. The flow strength was further increased to activate more slip systems to facilitate the deformation. Finally, the effect of grain size on the formability of pure Ti thin plate was verified by the forming process of bipolar plates for proton exchange membrane fuel cells (PEMFC).</p>