A method of manufacturing at least a root section of a wind turbine blade (10, 110, 310), the root section comprising a composite structure with a fibre-reinforced polymer material including a polymer matrix and fibre-reinforcement material, the method comprises the steps of: a) providing at least a first mould part (40, 140) for manufacturing at least a first root section part of said root section, the first mould part (40, 140) having a longitudinal direction and comprising a first mould surface (42, 142), the first mould surface (42, 142) defining at least a part of an outer surface of said first root section part manufactured in the first mould part (42, 142), the first mould surface (42, 142) having an end part (44, 144) with a first radius of curvature ( R 1), b) providing a mould inlay (50, 150) having an outer surface (52, 152, 252) and an inner surface (54, 154, 254) on top of the first mould surface (42, 142) with the outer surface (52, 152, 252) facing the first mould surface (42, 142) such that the mould inlay (50, 150) extends at least from the end part (44, 144) along a longitudinal section of the first mould surface (42, 142), the mould inlay (50, 150) having an outer surface (52, 152, 154) which at least at the end part (44, 144) has a radius of curvature corresponding to the first radius of curvature ( R 1) and the inner surface (54, 154, 254) has a second radius of curvature ( R 2) being smaller than the first radius of curvature ( R 1), and wherein the mould inlays comprise a proximal end and a distal end, the mould inlays being arranged so that the proximal end is arranged nearest the end part and the distal end is arranged farthest from the end part, and wherein the distal end of the mould inlay is tapered, c) providing fibre-reinforcement material (60, 62; 160, 162; 360, 362) and fastening means (64, 164, 264, 364) for fastening the root section to a hub of wind turbine on top of the mould inlay (50, 150) and the first moulding surface (40, 140), and optionally also sandwich core material, d) providing resin to the fibre-reinforcement material (60, 62; 160, 162; 360, 362), and e) curing the resin in order to form the composite structure. a) providing at least a first mould part (40, 140) for manufacturing at least a first root section part of said root section, the first mould part (40, 140) having a longitudinal direction and comprising a first mould surface (42, 142), the first mould surface (42, 142) defining at least a part of an outer surface of said first root section part manufactured in the first mould part (42, 142), the first mould surface (42, 142) having an end part (44, 144) with a first radius of curvature ( R 1), b) providing a mould inlay (50, 150) having an outer surface (52, 152, 252) and an inner surface (54, 154, 254) on top of the first mould surface (42, 142) with the outer surface (52, 152, 252) facing the first mould surface (42, 142) such that the mould inlay (50, 150) extends at least from the end part (44, 144) along a longitudinal section of the first mould surface (42, 142), the mould inlay (50, 150) having an outer surface (52, 152, 154) which at least at the end part (44, 144) has a radius of curvature corresponding to the first radius of curvature ( R 1) and the inner surface (54, 154, 254) has a second radius of curvature ( R 2) being smaller than the first radius of curvature ( R 1), and wherein the mould inlays comprise a proximal end and a distal end, the mould inlays being arranged so that the proximal end is arranged nearest the end part and the distal end is arranged farthest from the end part, and wherein the distal end of the mould inlay is tapered, c) providing fibre-reinforcement material (60, 62; 160, 162; 360, 362) and fastening means (64, 164, 264, 364) for fastening the root section to a hub of wind turbine on top of the mould inlay (50, 150) and the first moulding surface (40, 140), and optionally also sandwich core material, d) providing resin to the fibre-reinforcement material (60, 62; 160, 162; 360, 362), and e) curing the resin in order to form the composite structure.