Due to their unusual properties, the grain boundaries (GBs) with the lowest possible inverse density of coincidence sites = 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 = 3. Four different CSL facets (100) 3CSL , {110} 3CSL , {120} 3CSL and {210} 3CSL were observed simultaneously. Flat facets (100) 3CSL , {110} 3CSL , {120} 3CSL and {210} 3CSL form smooth edges (no slope discontinuity) with rounded rough GB portions. Rough surface curves away from the plane of the (-1, 2, 0) 3CSL facet at the edge with (-1, 1, 0) 3CSL facet as x β with β = 1.61 ± 0.09. At the edge between (210) 3CSL and (-1, 2, 0) 3CSL facets β = 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 β value depend on the details of the steps interaction at the vicinal surface. In our case the difference between measured β for two different facet edges can be due to the similar details of GB steps. C 2005 Springer Science + Business Media, Inc.