The time-correlated leakage characteristic of nonmetallic gaskets was analyzed. Considering the effect of the gasket material deterioration on the sealing performance, a time-correlated leakage predication model of nonmetallic gasket sealing connections based on the porous medium theory was proposed. Some tests were performed on compressed non-asbestos fiber gaskets, the leakage behavior was investigated and the coefficients in the leakage model were obtained by regression analysis of experimental data. The predicted leakage rates according to the model were compared with the data reported in some literatures, and the prediction accuracy was also validated. The presented time-correlated leakage model provides an effective way to predict the leakage rate and evaluating the life of gasket sealing connections.