Abstract: The invention relates to an apparatus for producing sideways curved wood wool fibers. The apparatus comprises a flat rotatable disc (2), in the surface of which is provided at least one knife assembly (7) comprising a thickness knife (8) which projects slightly above the surface of the disc (2) and which has a cutting edge extending substantially radially and further comprising width knives (9) positioned as seen in the direction of rotation of said disc (2), ahead of said thickness knife (8) and having a cutting edge extending substantially tangentially. The width knives (9) are positioned in the immediate vicinity of the thickness knife (8), whereas their cutting edge may engage the thickness knife (8). Further the apparatus is provided with at least one feeding device (5) for feeding the disc (2) with wood blocks (6) to be cut up into fibers. Further a run-up guide may be positioned ahead of the width knives.