Bone microdamage usually appearing in different type of microcracks in the bone matrix as a result of repetitive mechanical loading and can only be identified by microscopy.Histologic observation and quantitative analysis could be done based on the specific staining and labeling.The microdamage could be classified by their morphology and position related to the osteon.Bone microdamage is often accompanied with the decrease of bone strength and the increase of bone fragility.The length and location of the microcracks are the main factors to affect the biomechanical performance of bone.Microdamage accumulation will finally lead to bone fatigue fracture.