<p>To explore the size effect on the tensile deformation behavior and fracture mechanism of TA1 pure titanium (Ti) foils, the heat-treated specimens with different grain sizes are subjected to micro-tensile tests combined with scanning electron microscopy (SEM) and digital image correlation (DIC). The results indicate that the plasticity and flow stress decrease with the decreasing specimen thickness (<i>T</i>)-to-grain size (<i>D</i>) ratio, and the fracture morphology is determined by the value of <i>T</i>/<i>D</i>. Specifically, the larger the value of <i>T</i>/<i>D</i>, the more susceptible the material is to ductile fracture. Conversely, the smaller the value of <i>T</i>/<i>D</i>, the larger the area of the cleavage plane, which increases the susceptibility to brittle fracture. The local microorientation both at grain boundaries and within grains increases with grain size, showing enhanced orientation differences during deformation. During the initial plastic deformation stage, the degree of strain localization rises, thus increasing the surface roughness of the specimen. Macroscopically, the strength and plasticity decrease with decreasing <i>T</i>/<i>D</i>, contributing to crack propagation along the cleavage surface. Subsequently, as the deformation continues, fluctuations in local strain distribution increase, intensifying non-uniform deformation and leading to premature fracture.</p> Graphical Abstract <p></p>