Modeling studies of complex nonlinear systems by hybrid neural networks are described in this paper.The hybrid neural networks consist of linear dynamic neural networks and nonlinear static neural networks.Using such a hybrid neural network,the difficulties in training a single network can be decreased,and the solution for a nonlinear control strategy can be reduced to solving for a linear system.Thus,this method overcomes the shortcomings of long training time and lower accuracy observed for one single neural network.Both the serial and parallel neural networks have been used to model a CSTR.A comparison between the two hybrid neural networks on the basis of the results obtained is described.