Young\u27s modulus and the grain size have been measured with 10 kinds of polycrystalline ferromagnetic nickel-copper alloys annealed at 700°, 800°, 900°, 1000°and 1100℃. The following empirical relation has been found between Young\u27s modulus E and the average area of crystal grains S : E = E_0 - B log S, where E_0 and B are constants. Young\u27s modulus vs. composition curves for alloys with the same grain size were not always similar to one another, but every curve showed a minimum. The composition corresponding to the minimum shifted toward the nickel side as the grain size became large. With the addition of copper, Young\u27s modulus of nickel with comparatively small grains increased, whereas that with comparatively large grains decreased. These results suggest that the complication of the so-far as observed Young\u27s modulus vs. composition curves for nickel-copper alloys may be due partly to the difference in the grain size of the individual specimens. Finally, the measured results were compared with those calculated by the formula derived by Voigt and Reuss
