The lattice constant of thin metal films with body-centered cubic structure are investigated using the statistical moment method (SMM), including the anharmonicity effects of thermal lattice vibrations. The Helmholtz free energy, mean-square atomic displacements and lattice constant are derived in closed analytic forms in terms of the power moments of the atomic displacements. Numerical calculations for Fe, W and Nb thin films were found to be in good agreement with those of the other theoretical results and with the experimental data.