The mean value of the local magnetic moment M loc( T) of Invar alloys at a finite temperature T can be practically expressed by M 0 (1 - ST 2), where M 0 is the magnetization at 0 K and S is the coefficient of the T 2 term or the Stoner excitation term. The essential mechanism of the Invar effect is the volume contraction due to the decrease of M 2 loc( T) and not of M( T) 2. The magnetovolume coupling constant C has been found to be 1.1–1.3×10 −8 cm 6/ emu 2, almost the same value for Fe-base Invar alloys, by experiments on forced volume magnetostriction, thermal expansion coefficients and pressure effects on magnetization.