The development of air traffic during the last years, has greatly increased the density of aircraft in the airspace, and congestion on major airports. Indeed, on many airports, the taxi operation of aircraft between parking positions and runways, causes delays. The problem is increased by the development of hubs. In this article, a taxi optimisation tool using a Genetic Algorithm is introduced and tested on Roissy Charles De Gaulle Airport. The tool can help choosing the best taxiways to reduce the time spent from the gate to the runway or the runway to the gate, respecting the separation with other aircraft. It can also help choosing one way taxiways regarding to traffic and wind, and also measuring the impact of opening a new taxiway or closing an existing taxiway. Simulations are presented on a one day traffic at Paris Roissy. Delays are correlated to the traffic density on the airport.