Abstract: In order to reveal the toughening mechanism of bamboo, microtensile test combined with scanning electron microscopy (SEM) was used to observe its insitu crack propagation process during tension. Furthermore, high resolution field emission gun (FEG) SEM was applied to study the consequent fracture surface with different magnification. The results showed that the microcrack in the bamboo followed the way of “zigzag” during longitudinal tension, in addition to the frequently observed delamination located between vascular bundles and parenceymal cells, among bamboo fibers or parenchymal cells, and even between the lamellae in the cell wall of fibers or parenchymal cells. All the above observations pointed to the existence of multiscale weak interface from tissue, cell to subcellular level in bamboo, which could increase the toughness of bamboo by blocking the growth of cracks and maximizing the work to fracture.