Abstract: The present calculation theory and method are insufficient to analyze the service lifetime of structural members for existing hydraulic steel gate structures. To overcome these disadvantages, based on the principl es of reliability, minimal reliability criterion was determined to predict the service lifetime. A method for service lifetime prediction was proposed on the basis of the reliability theory. Furthermore, approximate formulae to predict the service lifetime were derived using the reliability theory. Finally, some numerical examples were presented to illustrate the proposed method. The results indicate that the proposed method is very effective in predicting service lifetime of structural members for existing hydraulic steel gate structures.