A hydraulic hinge for a glass door has a fixing plate, a rotating seat, a pivot and a buffer module. The rotating seat is combined with the fixing plate and has a first oil passage and a second oil passage. The pivot is combined with the fixing plate and the rotating seat. The buffer module is mounted in the rotating seat and has a valve having a circular groove selectively communicating with the second oil passage. A glass door can be pivoted relative to the rotating seat. The valve is structurally simple, thereby simplifying the manufacturing process. With the valve and the second oil passage, the velocity of closing the glass door gradually increases from low speed to high speed.