It is suggested that osteocytes play an important role in bone metabolism, especially in their regulation by mechanical stimuli. However, it has been still discussed how the osteocytes were activated by mechanical stimuli. Recently, theories based on fluid flow-induced shear stress stimulation around osteocytes in bone have appeared with the most prominence. In addition, we would like to introduce several theories to mechanically stimulate osteocyte, such as deformation, strain amplification by radial hoop strains, flow induced integrin mediated signaling, and primary cilia mediated signaling. To validate the availability of these theories, it is important to obtain 3-dimensional morphometrical data of osteocytes as well as their interaction with pericellular matrix.