The problem of cooperation is essentially captured by the Prisoners’ Dilemma (PD) game.However,people often cooperate in social dilemmas.Two important explanations for the evolution of cooperation are directed reciprocity in repeated PD and spatial reciprocity by assorting cooperators.In the experimental studies,humans are indeed highly cooperative in repeated games and use conditional strategies.So we are here to investigate the effects of the simplest conditional strategy--Tit for Tat in promoting cooperation.