Abstract: The pressure-dependent elastic properties of the Fe–S system are important to understand the dynamic properties of the earth’s interior. We have therefore under-taken a first-principles study of the structural and elastic properties of FeS2 polymorphs under high pressure using a method based on plane-wave pseudopotential density function theory. The lattice constants, elastic constants, zero-pressure bulk modulus, and its pressure derivative of pyrite are in good agreement with the previous experiments and theoretical approaches; the lattice constants of marcasite are also consistent with the available experimental data. Calculations of the elastic constants of pyrite and marcasite have been determined from 0 to 200 gPa. Based on the relationship between the calculated elastic constants and the pressure, which can provide the stability of mineral, it would appear that pyrite is stable, whereas marcasite is unstable when the pressure rises above 130 gPa. Static lattice energy calculations predict the marcasite-to-pyrite phase transition to occur at 5.4 gPa at 0 K.