The instability of chemically reactive systems is closely related to potential hazards, This instability appears when a system is nonlinear and far from thermodynamic equilibriuln, The stability of a chemica11y reactive system is analysed on the basis of non-equilibrium thermodynamics in this paper.The heat conduction and chemical reactions are treated together in terms of entropy production.The stability criterion of a chemically reactive system is derived being consistent with conventional one.The present theory elucidates that the instability results from the negative resistivity of reaction rate to affinity inherent in a chemically reactive system.