Steady states of a complete mixing type reactor in which exothermic and endothermic reactions proceed simultaneously Vary considerably, depending on the thermal characteristics of the reactions and operating conditions of the reactor. In this study, a simplified method to predict multiplicity and stability of steady states is established. Shapes of a function defined as the sum of products of heat of reaction and reaction rate constant are classified into 6 types, and a number of steady states are predicted based on the relation of location between 2 isoclines on which trajectories are horizontal and vertical. The stability of steady states is determined based on the directions of trajectories in each region separated by 2 isoclines. By making use of the method, outlines of the region in which steady states exist, stability of steady states, and also safe operating region can be predicted.