Abstract: The present invention relates to a method for manufacturing of a stiffening element (1, 1?, 1?, 2) of composite material, which stiffening element is provided for attachment to a curved shell surface (9), said stiffening element (1, 1?, 1?, 2) comprises a web (3) and at least one flange (5), the outer surface (11) of which has a radius (R) of curvature corresponding with the curvature of said curved shell surface (9) The method comprises the following steps of: providing a substantial flat blank (19) of composite material; and folding said blank (19) along a first folding line (13), the curvature of which is defined by an imaginary three-dimensional intersection line (115) being derived from the radius (R) of curvature of the curved shell surface (9).