In　this　paper,　we　consider　the　diffusion　limit　of　the　small　mean　free　path　for　the　radiative　transfer　equations,　which　describe　the　spatial　transport　of　radiation　in　material.　By　using　asymptotic　expansions,　we　prove　that　the　nonlinear　transfer　equation　has　a　diffusion　limit　as　the　mean　free　path　tends　to　zero,　and　moreover　we　study　the　boundary　layer　problem　and　mixed　layer　problem　in　bounded　domain　[0,1].　Then　we　show　the　validity　of　their　asymptotic　expansions　relies　only　on　the　smoothness　of　boundary　condition,　and　remove　the　Fredholm　alternative　and　centering　condition.