To provide a frame structure of a motorcycle easy in work of a welding part to a tank rail of a seat rail front end part and capable of enhancing welding strength to the tank rail of the seat rail front end part.

A bracket 34 is welded to a downward bending part of the single tank rail 8 extending to the rear of a car body from a head pipe 2, and respective front end parts of a pair of left and right seat rails 10 are welded to flat both side surfaces of the bracket 34 to constitute this frame structure of the motorcycle. Since the respective front end parts of the pair of left and right seat rails 10 are welded to the flat both side surfaces of the bracket 34 welded to the tank rail 8, even when the seat rails 10 and the tank rail 8 are composed of a round pipe material, a welding part (a tip part) can be welded in close contact to the side surfaces of the bracket 34 by working the welding part (the tip part) in a plane shape to the tank rail 8 of the respective seat rails 10 to facilitate the work of the welding part to the tank rail 8 of the front end parts of the seat rails 10 as well as to improve the welding strength.