The sealing mechanism of flat copper gaskets and ConFlat (CF) flanges is studied. Finite element (FE) modeling was used and the results were validated through experiments. Only a part of the CF flange knife surface contributes to the sealing. A bilinear character of compression force and displacement evolution was numerically and experimentally identified and has been described by a phenomenological formula. The influence of the flange collar and gasket material properties on the sealing mechanism was studied. The plastic strain field produced by the knife in the gasket was compared to microhardness tests. The possibility of crack propagation was studied because of large plastic deformation (FE results) near the knife tip.