A novel fabrication method of silicon quantum wire gate-all-around transistor (GAAT), in which the gate oxide and the gate electrode are wrapped around ultrafine silicon quantum wire, has been proposed. In order to verify one-dimensional (1D) subband effects, we have studied quantum transport in Si quantum wire GAAT with a width of 50 nm at 1.5 K in zero-magnetic field and in fields up to 10 T. Electrical population and magnetic depopulation of 1D subbands are clearly observed.