The samples of Phyllostachys edulis were collected at different growth periods. Using polarizing microscope and the image processing technique, the variation law of crystal area in bamboo vascular bundle on cross section was studied. The results showed that the crystal area proportion of thick cell wall increased linearly during early bamboo growth. It decreased due to bamboo lignifications and its cellulose proportion reduced when bamboo entered to the middle age. It varies slowly and keeps invariant due to bamboo lignification decrease during later bamboo growth.