A structure for coupling roof rails using a side two-buck structure includes a floor formed of a bottom of a vehicle body, side inner panels coupled to both side surfaces of the floor, and roof rails coupled to upper portions of the side inner panels. A method for coupling roof rails includes coupling side inner arms to side surfaces of a vehicle floor, coupling a cross shaped joint part to an upper portion of the side inner arm, coupling a side inner reinforcement part to outer surfaces of the side inner arm and the joint part, coupling the roof rail to an upper portion of the side inner reinforcement part and an upper surface of the joint part, coupling a side outer reinforcement panel to an outer portion of the side inner reinforcement part, and coupling a side outer panel to an outer portion of the side outer reinforcement panel.