The present invention relates to a method of determining the flight parameters and fuel consumption of at least one phase of flight of an aircraft. This method is according to invention characterized in that it comprises, for said or each flight phase, the following steps: - a step of defining a plurality of said flight phase considered embodiments each of a set of successive segments (Sn) having their own flight parameters (ISn) predetermined or determined randomly or pseudo-randomly, - a step of calculating, for each embodiment of said flight phase considered, on the basis of aerodynamic characteristics (IAV) of said plane and the flight parameters (Isn) of each segment (Sn) of the embodiment under consideration, the speed (Vn) of the segments (Sn) and fuel consumption (CFPM) of said aircraft to said embodiment, - a step of optimizing that are retained as flight parameters of the flight phase considered, that the flight parameters (IFPopt) of the segments which constitute the embodiment of the said flight phase to wherein the fuel consumption (CFPopt) is minimized, and the speed (VSnopt) of the segments of said embodiment and said fuel consumption (CFPopt) resulting. The present invention also relates to a system for implementing said method.