In order to accomplish efficient execution of a dynamic simulation of engineering systems, it is improtant to construct suitable mathematical models for the target simulation. In the construction of the mathematical models, it is necessary to estimate the system behavior to supporse the phenomena which are needed to be modeled. The outline of system behavior in a real mathematical model selection procedure is estimated from the similar cases as a target simulation case which are stored as our experience or in literature. By basing on the consideration, the case-based reasoning is applied in this study. The system behavior of a target simulation is estimated by refining that of the retrieved case from case base. In a simple estimation trial of plant behavior of pressurized water reactor nuclear power plants at a small break loss of coolant accident, the outline of plant behavior is estimated in reasonable accuracy.