The　method　of　upper　and　lower　solutions　and　the　Schauder　fixed　point　theorem　are　used　to　investigate　the　existence　and　uniqueness　of　a　positive　solution　to　a　singular　boundary　value　problem　for　a　class　of　nonlinear　fractional　differential　equations　with　non-monotone　term.　Moreover,　the　existence　of　maximal　and　minimal　solutions　for　the　problem　is　also　given.