<p>Many macro-scale studies have reported that heterogeneous structured metals can achieve superior synergy of strength and ductility. However, there is a lack of research on the fracture toughness of heterogeneous metals, especially at the micro-scale. In this study, the influence of grain thickness and sample size on the crack propagation behavior of gradient lamellar nickel was explored in micro-cantilever beam samples with pre-notches by in-situ bending testing under scanning electron microscopy. The results showed that samples with large-size were completely blunted due to relatively uniform plastic deformation behavior. However, the small-size samples exhibited crack propagation behavior, and the strain localization near pre-notches of small-size samples with large lamellar thickness was high since no sufficient grain boundary hindered the crack growth. Moreover, all the large-size samples were more resistant to crack propagation than the small-size samples, probably because the stress field of the former was close to the plane strain state.</p>