Humans　are　social　animals　and　need　to　cooperate　to　survive.　However,　individuals　are　not　cooperative　in　every　social　interaction,　and　their　cooperation　may　depend　on　social　context.　The　present　study　used　a　social　dilemma　game　to　investigate　whether　an　opponent＇s　tendency　to　be　cooperative　over　time　influenced　a　player＇s　behavior　and　neural　response　to　outcomes　in　the　game.　University　students　(＂players＂)　thought　they　were　playing　against　other　students　(＂opponents＂)　in　the　Chicken　Game　but　were　actually　playing　against　a　programmed　computer.　Participants　were　randomly　assigned　to　play　with　an　opponent　who　tended　to　be　competitive　(cooperative　20%　of　the　time)　or　who　tended　to　be　cooperative　(cooperative　80%　of　the　time).　The　results　showed　that　early　in　the　game,　participants　in　both　groups　adopted　a　＂tit-for-tat＂　strategy.　However,　as　the　game　progressed　and　the　opponent＇s　behavioral　tendency　became　more　noticeable,　players　in　the　competitive-opponent　group　became　generally　more　cooperative　to　limit　their　losses.　ERPs　analyses　indicated　that　players　had　a　higher　P300　and　larger　theta　power　in　response　to　the　opponent＇s　aggression　but　not　to　the　opponent＇s　cooperation　when　their　opponent　showed　a　tendency　to　be　cooperative　vs.　competitive.　The　results　suggest　that　people　adjust　their　cooperative　behavior　based　on　their　opponent＇s　behavior　in　social　interaction,　and　aggression　captures　more　attention　than　cooperation　in　this　process.