Design techniques for non-linear dynamic systems are closely related to their stability properties. Stability results can be used to design a reliable controller. This paper discusses the stability analysis of the dynamic neural network control (DNNC). The results from DNNC stability analysis will be used to define the neural network stability index (NNSI). The NNSI is a practical index which in current form can only be used with DNNC structures. The NNSI can be used to determine the optimal DNNC network structure. In addition, we will provide guidelines for the design of an optimal DNNC network structure for the conventional neural network structure for model-based control strategies. In this study, DNNC will be designed for a non-isothermal CSTR as an example of a wide class of non-linear processes.