A mount bracket includes a fixing frame, an installation unit, a movable member, a positioning member and a release member. The fixing frame includes a side wall and an end wall. The movable member has a first board movably connected to the side wall and a second board. The first board includes an engaging hole and a contact portion. The positioning member is fixed to the side wall and has an engaging portion. The movable member is movably connected to the first board. When in assembling, the contact portion and the second board contact the inside and outside of a post of a rack. The fixing frame is pushed toward the post to insert the installation unit into the holes of the post. The engaging portion of the positioning member is engaged with the engaging hole of the movable member to connect the movable member to the fixing frame.