Abstract: Intelligent techniques，such as artificial neural network，fuzzy inference logic and expert system，are used in several applications. These techniques are based on human intelligence and way of thinking. They do not need much system information. New ideas and methods are presented for resolving the key problems of complex control system. The intelligent techniques were used in ethylene plant. The intelligent system structure and application were demonstrated in thermal cracker，and the application results in the process were shown. The successful application illustrates that the intelligent techniques are very attractive in process control.