A method of assembling a sub-frame of a vehicle with the front side member of the vehicle includes integrally forming a supporting end of a mounting nut that protrudes in a radial direction in a center of an external circumference of the mounting nut, forming a through hole in a lower surface of the front side member, forming a seat as a groove in an internal surface of the through hole, inserting a lower end of the mounting nut into the through hole so that the supporting end is supported by a circumference of the through hole, settling a head of the mounting nut in the seat, and welding the supporting end and the head along the circumference of the through hole and a side of the seat to assemble the sub-frame to the front side member.