A first-principle study of Fermi surface and magnetism for antiferromagnet UPtGa₅ is done by all-electron band calculations using fully-relativistic spin-polarized LAPW method in a local spin-density approximation. The Fermi surfaces, mainly having cylinder-like shapes with open structure along the [001] direction, explain well the angular-dependence of de Haas-van Alphen frequencies. In a picture of 5f-band antiferromagnet, the local magnetic moment at U site is in good agreement with the observed moment.

PACS numbers: 71.18.+y, 71.20.--b