The effects of surface relaxation in the powder diffraction pattern from metal nanoparticles are discussed. Molecular dynamics simulations are carried out to simulate the structure of a series of free-standing Al and Cu nanoparticles of different sizes and stabilization temperatures. The diffraction patterns found from considering the average atomic positions are then modeled, assuming different forms for the effects of the surface strain field. The modeling finds that the strain field in the simulated Al particles does not result in an appreciable effect on the peak broadening. However, that of the Cu particles results in anisotropic peak broadening, which is not able to be properly accounted for by the existing isotropic surface strain models.