The relationship between the changes of microstructure and mechanical behavior of Moso Bamboo （Phyllostachys pubescens） with large compressive deformation were determined with a microscope-loading device. Bamboo samples were compressed in three directions: radial, tangential and longitudinal. The results indicated that: 1) under radial and tangential compression, parenchymatous ground tissue was the basic component for transferring loads. It exhibited the same yield performance and, therefore, bore the same yield limit. In the same way, the bamboo wall revealed the same macro mechanical behavior, which showed bamboo could be regarded as a two-dimensional composite material. 2) Under longitudinal compression, the loads were borne by bamboo fibers and, therefore, the yield limit of bamboo with longitudinal compressive deformation is three times higher than that with transverse compressive deformation.