Abstract: Method and system for determining the longitudinal position of the gravity center of an aircraft having an adjustable stabilizer. At a flight point for which the aircraft is in equilibrium with its elevator in aerodynamic prolongation of the adjustable stabilizer, the gravity center position is calculated from the position (X.sub.F) of the focus (F) of the aircraft (1), the measurement of the deflection (iH) of the adjustable stabilizer, a first function K(M, N1) of the engine speed (N1) of the engines of the aircraft and the Mach number (M). The first function is representative of the deflection value (iHRef) of the adjustable stabilizer. In the case where the gravity center would be at the focus (F) of the aircraft, a second function H(Cz, M) of the lift coefficient (Cz) of the aircraft and of the Mach number (M) is used. The second function is representative of the stabilizer deflection variation for a variation of 1 percent of the gravity center position.