An apparatus for fastening a pressure vessel of a vehicle is arranged to fix the pressure vessel to a vehicle body, where the pressure vessel includes a cylindrical body, dome-shaped curved portions at both ends of the cylindrical body, and bosses at distal ends of the curved portions. The apparatus is arranged to receive the pressure vessel, and further includes: elastic bodies disposed along circumferences of the curved portions of the pressure vessel to support surfaces of the curved portions and to be disposed on a virtual flat cross section vertically crossing the curved portions along the circumferences of the curved portions to support the curved portions of the pressure vessel in all directions; and a fixed structure having one side connected to the elastic bodies and the other side connected to the vehicle body.