A device for the determination of a plane containing the mechanical axis of the femur includes a mechanical member of articulated and or/sliding elements for the determination and memorization of two positions in space (F1 and F2) of the same point (F) of the knee, freely selected, when the knee is in position P1 and P2, relative to a referential system. These positions are obtained by rotation of the inferior limb from position P1 to position P2, about the centre of the femoral head. The orientation of an omega plan perpendicular to the frontal or sagittal plane containing the centre of the femoral head and the centre of the knee can be materialized by a rod contained in this plane, which rod is fixed perpendicularly and in the middle of the arm having at the ends two localization elements for points F1 and F2.