The effect of pressure on the equilibrium isotope fractionation is examined theoretically. The calculation technique is developed, using the quasi-harmonic approximation for solids. The formula connecting the pressure derivative of the β-factor (reduced isotopic partition function ratio) and its temperature derivative is obtained. The application of the technique to some silicates (quartz, albite, enstatite, pyrope, grossular, forsterite), rutile and calcite ( 16O- 18O fractionation), graphite-calcite-diamond ( 12C- 13C fractionation) and brucite-water (H-D fractionation) shows that pressures of tens of kbars may produce a measurable effect on the equilibrium isotope fractionation and even change the sign of the isotopic shift. Estimation of the pressure effect is important for correct interpretation of high pressure experimental isotopic data and for correct calculation of equilibrium isotopic constants.