A seatback frame structure is provided for the manufacture of a seat of a vehicle. The seatback frame structure includes a pair of side brackets and an upper cross member. The side brackets are coupled at lower ends thereof to left and right sides of a seat frame. The upper cross member is transversely mounted between the upper ends of the side brackets, and a headrest is placed on the upper portion of the upper cross member. Each of opposite ends of the upper cross member comprises a downwardly inclined surface, and is coupled to the upper end of a corresponding side bracket.