<p>Bamboo is widely used as a functional and natural material in Japan and several other Asian countries. The novel mechanical characteristics of bamboo are its high toughness, stiffness and lightness. Bamboo's hollowness enables it to be light to grow faster. However, by being light, bamboo indeed becomes vulnerable to strong winds and can also fail to support its own weight. To overcome this limitation, the woody parts of bamboo are reinforced with thin but tough fibers (vascular bundles). Each fiber is considered to be as rigid as steel. This fact makes us realize that bamboo is one of the wonderful naturally-occurring functionally graded materials. From this point of view, we have investigated the relationship between vascular bundle distribution and bending properties of bamboo. It is known that a cross-section of bamboo revealed the non-uniform distribution of fibers in the woody parts. The density of the fibers progressively turns out to become thicker from the inner to the outer surface, pointing out that the outer parts are stiffer and stronger than the inner parts. This shows that bamboo improves their bending strength by appropriately adjusting the distribution of their fibers. Our present study could help develop advanced materials by mimicking the bamboo model for its lightness and toughness.</p>