The fractal dimension of the contacting surface of friction pair has significant influence on the sealing behavior and friction characteristics of mechanical seals.According to the contact fractal model and leakage fractal model of the end face of the friction pair，the optimum fractal dimension，beneficial to alleviating the wear of the end face and the optimum fractal dimension related to the allowable leakage rate were studied respectively.The relationship between elastic contacting area ratios of NHM70 type mechanical seal Are/Ar and the fractal dimension D，as well as the relationship between leakage rate q and fractal dimension D，were obtained by numerical calculation.Experimental studies on NHM70 type mechanical seals were also carried out on a self-designed testing device.The theoretical calculation and the experimental studies showed that the optimum fractal dimension of the soft ring surface of NHM70 type mechanical seal was 1.61 after comprehensively considering the wear rate，leakage rate and manufacturing cost.