This paper presents the results of an experimental research on the properties of “It” gaskets, which are used to ensure the tightness of flanged joints at pressured vessels. In this work, were considered the vessels working at variable pressure and high temperature. Experimental data are used to determine a general relation for calculating the endurance reduction factor of the flanged joints. This factor is recommended for determining the safe lifetime of the gasket, which is the most sensitive constructive element of flanged joints.