The non-isothermal chemical and petrochemical processes represent a particular technological position in exploitation, by a possible thermal instability evolution. The aim of our paper is to analyse the stationary regime’s stability, for the Non-isothermal Continuous Stirred Tank Reactor- NCSTR, to guaranty the imposed security and performances by an adequate control solution. The mathematical nonlinear model based on the mass and thermal balance is estimated. From the thermal balance equation corresponding to the stationary operating point, the heat characteristics for the stability analysis and the necessary geometrical conditions of internal stability for NCSTR are deduced. The nonlinear dynamic state model is linearized around the stationary point and the stable operating specifications are inferred. To demonstrate the necessary and sufficient conditions for the internal stability of the exothermal regime of NCSTR, the Lyapunov stability theorem for the linear systems is established.