A roof antenna mounting unit for a vehicle is provided. The roof antenna mounting unit is mounted in a mounting aperture formed in a roof of a vehicle and fixes an antenna. The mounting unit includes a fastening member having a molding portion and a fastening portion that is inserted into the mounting aperture. An upper cover is integrally coupled to the fastening member by injection molding to surround an outer circumferential surface of the molding portion and is disposed at an upper side extraneous to the roof. A sealing member is interposed between the roof and the upper cover and a fixing nut is mounted on the fastening portion inserted into the mounting aperture at a lower side in the roof.