In this paper, we consider a Riemann-Liouville type two-term fractional differential equation boundary value problem. Some positive properties of the Green's function are deduced by using techniques of analysis. As application, we obtain the existence and multiplicity of positive solutions for a fractional boundary value problem under conditions that the nonlinearity f (t, x) may change sign and may be singular at t = 0, 1 and x = 0, and we also obtain the uniqueness results of positive solution for a singular problem by means of the monotone iterative technique.