Experimental observations and theoretical investigations exhibit that a group-IV(V) transition metal diboride (0001) surface is terminated with a 1 x 1 TM(B) layer. As to a group-III transition metal diboride, we have investigated the stability boundary of ScB2 (0001) surfaces using first principles total energy plane-wave pseudopotential method based on density functional theory. The Mulliken charge population analysis shows that Sc atoms in the second layer cannot provide B atoms in the first layer with sufficient electrons to form a complete graphene-like boron layer. We also found that the charge transfer between the first and the second layer for the B-terminated surface is more than that for Sc-terminated surface. It elucidates the reason that the outermost interlayer spacing contract more strongly in the B-terminated surface than in the Sc-terminated surface. The surface energies of both terminated ScB2 (0 0 0 1) surfaces as a function of the chemical potential of B are also calculated to check the relative stability of the two surface structures. (C) 2011 Elsevier B.V. All rights reserved.