Dilatation of one side of the pupil can be caused by eye diseases or central nervous system diseases, as follows: 1, eye diseases: including eye trauma, injury to the motor nerve, there may be eye diseases, such as glaucoma or ophthalmic nerve diseases; 2, central nervous system diseases: common brain trauma affecting the eyes or the central nervous system brain hemorrhage, cerebral infarction leading to brain hernia, which also will be Unilateral pupil dilation will also occur. These two problems need to go to the hospital, by the specialist doctor to determine which kind of problem, and then active symptomatic treatment.