In this paper, we consider a Cauchy problem for nonlinear fractional differential equation with constant coefficient λ > 0 of the type:We present monotonicity and continuity of two-parameter Mittag-Leffler function E α,β (z) for z > 0. By using monotonicity, continuity, asymptotic properties of E α,β (z) for z > 0 and fixed point theorems, we obtain existence of solution. Finally, two examples are given to illustrate our theoretical results.