A mounting bolt for a sub-frame is disclosed. A mounting bolt for a sub-frame that is used to engage a sub-frame to a front side member of a vehicle according to one or a plurality of exemplary embodiments of the present invention may include a bolt body that is inserted into a penetration hole of a mounting bracket fixed on the front side member and is joined to the mounting bracket through a first flange that is formed at an upper end circumference thereof, a support body at which a second flange is formed at a lower end circumference to have a hollow space and is disposed on the bolt body, and a joining plate that is interposed between the first flange and the second flange and is welded with a first join protrusion that is formed on an upper surface of the first flange and a second join protrusion that is formed on a lower surface of the second flange by electrical resistance.