The conception of justice as fairness elaborated by John Rawls state the consideration about which terms of cooperation people consider as reasonably just to offer one another having in mind the society as a cooperative scheme for mutual advantage. The first idea is one that all people have a right to an equal set of primary social goods which makes possible for each one of them pursue their respective life plans. Nevertheless Rawls notes that people are exposed since they are born to natural and social contingencies and that mental and physical talents and abilities and conditions of wealth and social status are arbitrarily distributed. This affects profoundly peoples perspectives of life. Rawls afirms that is necessary regulate this scene of morally arbitrary inequalities and so he elaborates two principles of justice. The first - the equality principle - guarantees the right to an equal and most extensive system of liberties to all people. The second - the difference principle - admit economic inequalities only if these benefit especially those who are less favoured by the natural and social lucky. In the Rawlsian Society of Peoples there are different levels of wealth among peoples. So, the international justice must establish principles which struggle these inequalities so as to allow burned peoples to join well ordered peoples in the reasonably just Society of Peoples. It is the duty of assistance which makes it possible. So, the Rawlsian conception of justice as fairness is maintained when the law of peoples is elaborated so that the purpose to be reached by the difference principle in the closed system society is also reached in the Society of Peoples by the duty of assistance. In both cases the less natural and socially lucky persons and peoples rely on a principle which guarantee them a social condition sufficiently necessary that make possible for them to pursue with dignity and self-respect their life plans.