Horizontal tail plane of an aircraft which comprises a single torsion box which also comprises a front spar (1), a rear spar (2), an upper cover and a lower cover, ribs and two closing ribs (3), the front spar (1), the rear spar (2) and the upper and lower covers extending between said two closing ribs (3) and along the whole span of the horizontal tail plane in a symmetric layout with respect to the symmetry axis of the aircraft.