Microvasculature in the periodontal ligament has important roles in the remodeling of periodontal tissue in orthodontic tooth movement. We have been observing morphological and functional reactions of the microvasculature to mechanical forces using an in vivo animal model. We observed that the microvasculature sensitively reacts to mechanical stimulation including alteration of hemocyte behavior in blood flow, which is closely related to bone remodeling. I will show these reactions of the microvasculature to mechanical pressure and its change with aging, and I would like to present discussion about the optimum orthodontic force depending on the age of the patient.