Mittag-Leffler function is very useful in the theory of fractional differential equations. Ostrowski's inequality is very important in numerical computations and error analysis of numerical quadrature rules. In this paper, Ostrowski's inequality via generalized Mittag-Leffler function is established. In application point of view, bounds of fractional Hadamard's inequalities are straightforward consequences of these inequalities. The presented results are also contained in particular several fractional inequalities and have connection with some known and already published results.