A mounting apparatus of a side airbag for a vehicle is advantageous in that in the event of a side crash, a deployment direction of an airbag cushion is guided to a slit of a seat pad by an inner frame and an outer bracket, so that the airbag cushion is distended and deployed towards a front of a seat back, thus allowing an occupant sitting in a seat to be more safely protected via the deployed airbag cushion.