A rail assembly for a glass door has a rail body, two clamping members, a pushing member and at least one pushing bolt. The rail body has a clamping channel defined in and along the rail body. The clamping members are movably mounted in the clamping channel. Each clamping member has a clamping recess and an inclined guiding surface. The clamping recess is defined in the clamping member at a side facing to the other clamping member. The inclined guiding surface is defined in the clamping member at a side opposite to the other clamping member. The pushing member is mounted movably in the clamping channel, holding the ends of the clamping members inside and has two inclined surfaces abutting respectively with the guiding surfaces of the clamping members. The least one pushing bolt is screwed into the rail body and pushes against the pushing member.