The sedimentation of isotope atoms hits been realized in monoatomic liquid and solid Se. It was observed that the concentration ratio Se-82/Se-76 increased by % level in the specimen ultracentrifuged at 0.7-0.9 million G under liquid and solid states. The present result is evidence of sedimentation of substitutional atoms in condensed matter via self-diffusion, and suggests its possible application to isotope separation by condensed matter centrifugation.