Cornea is one of the important structure for the eye. It is transparent and locats about 1/6 before fiber membrane. Collagen arrangement is orderly and lets the light through. Because of the special arrangement of collagen, it makes transparent state. Thus it can be seen the important role of corneal collagen. Now we reviewed the distribution of collagen, physiological function, changes of corneal collagen cross-linking, clinical use and security as follows.