Due to their unusual properties, the grain boundaries (GBs) with the lowest possible inverse density of coincidence sites Sigma=3 play a special role in the GB engineering. The as-grown shape of the cylindric tilt grain boundary (GB) in Nb bicrystal grown by the floating zone method has been studied with the electron back-scattering diffraction method. Both grains form the superlattice called coincidence site lattice (CSL) with the lowest possible inverse density of coincidence sites Sigma=3. Four different CSL facets (100)(Sigma3CSL), {110}(Sigma3CSL), {120}(Sigma3CSL) and {210}(Sigma3CSL) were observed simultaneously. Flat facets (100)(Sigma3CSL), {110}(Sigma3CSL), {120}(Sigma3CSL) and {210}(Sigma3CSL) form smooth edges (no slope discontinuity) with rounded rough GB portions. Rough surface curves away from the plane of the (-1, 2, O)(Sigma3CSL) facet at the edge with (-1, 1, 0)(Sigma3CSL) facet as beta with = 1.61 +/- 0.09. At the edge between (210)(Sigma3CSL) and (1, 2, 0)(Sigma3CSL) facets beta = 1.46 +/- 0.09. Both values reveal the GB roughening belonging to the Pokrovsky-Talapov universality class. It has been shown for Pb surfaces [K. Arenhold, S. Surnev, P. Coenen, H.P. Bonzel and P. Wynblatt, Surf. Sci. 417 (1998) L1160] that the beta value depend on the details of the steps interaction at the vicinal surface. In our case the difference between measured beta for two different facet edges can be due to the similar details of GB steps. (C) 2005 Springer Science + Business Media, Inc.