A mounting unit for a sub-frame of a vehicle is provided. The mounting unit is used for engaging a front side member with the vehicle's sub-frame. The mounting unit includes a welding seat that is formed as a groove on a lower side of the front side member. The mounting unit further includes a mounting nut assembled with the front side member, a welding flange integrated with an external circumference and a head of the mounting nut and inserted into the welding seat to be combined with the welding seat, and a member seat that is formed outside the welding flange and with which a fender upper connecting member is combined.