Stability of impulsive fractional-order systems is investigated in this paper. By Lyapunov direct method and vector comparison principles, results about asymptotic stability are obtained. To this end, comparison principles, including scalar and vector forms, are generalized to impulsive fractional-order systems, through which fractional inequalities are derived for the linear impulsive systems. Then, based on such comparison principles, sufficient conditions for the MittagLeffler stability of impulsive fractional-order systems are established. Examples are given to show the effectiveness of the results.