In this paper, we give a criterion on instability of an equilibrium of a nonlinear Caputo fractional differential system. More precisely, we prove that if the spectrum of the linearization has at least one eigenvalue in the sector n 2 C n f0g : j arg j 2o;where 2 (0; 1) is the order of the fractional differential system, then the equilibrium of the nonlinear system is unstable.