A simplified model of interface roughening is presented. As a special case, the roughening process of a grain boundary has been simulated. This process is based on random walks of atoms across the grain boundary with a restoring effect induced by neighbouring atoms to reduce the roughness. The root-mean-square roughness σ decreases with the increasing restoring effect. The Hausdorff dimension of the rough interface increases from 2.1 to 2.45 with the decreasing restoring effect. The rough interface can be also described by the multifractal spectra. The results show that the larger the restoring effect, the wider the singularity spectrum.