Magnetostriction constants &lgr;111have been determined at room temperature with the double‐crystal x‐ray diffraction technique by measuring a small change in the lattice parameter. Measurements were carried out for epitaxial garnet films of Y3Fe5O12and Sm0.85Tm2.15Fe5O12grown on [111] oriented Gd3Ga5O12substrates; and also for flux‐grown bulk single crystal of Y3Fe5O12. Values of &lgr;111obtained were −1.7×10−6for film and −3.0×10−6for bulk crystal. &lgr;111of Sm0.85Tm2.15Fe5O12was −5.0×10−6. The discrepancy in &lgr;111values between film and bulk Y3Fe5O12is almost accounted for by the fact that the film can deform only perpendicular to the film plane due to the shear force of the substrate. It has been experimentally demonstrated that &lgr;111does not depend on film thickness or lattice mismatch, which is consistent with the present analysis.