Thermal stability analysis is imperative to ensure the safe operation of chemical reactors carrying out highly exothermic reactions. We present a thermal stability study for a fixed bed gas oil hydrotreating reactor working at industrial operating conditions, which involves three phases, 15 lumped hydrocarbon chemical families (saturates, olefins, sulfur containing compounds, triaromatics, diaromatics, and monoaromatics), and distributed in 3 boiling point cuts to differentiate light and heavy hydrocarbons. The influence of various system parameters on the reactor thermal stability has been investigated using the sensitivity-based Morbidelli-Varma criterion. It is found that most of the operating parameters can be used in principle to tune the reactor thermal stability, except for the inlet feed temperature, which, though shifting the occurrence of runaway to different parameter regimes, does not improve the reactor thermal stability behavior. In addition, the convective heat removal plays an important role in changing the reactor thermal stability. The obtained results are given in various operating parameter planes, which are divided by the criticality curves for parametric sensitivity into three regimes, LTO (low-temperature operation), HTO (high-temperature operation), and M (multiplicity), thus forming various reactor operation diagrams, which can be used to guide the reactor design and operation.