We examine the mechanical properties of single-crystal and ultrananocrystalline diamond (UNCD) by simulating their fracture using semempirical quantum mechanics and density functional theory. Our results predict a failure strain of 0.13 and a fracture stress of 100 GPa for UNCD, which are 37% and 43%, respectively, that of single-crystal diamond. The Young's modulus of UNCD is E = 1.05 TPa which is only slightly smaller than that of single-crystal diamond (E = 1.09 TPa). The UNCD fracture stress value (sigma(f) = 100 GPa) is very large compared to that observed experimentally (sigma(f) < 5 GPa). We use Griffith theory to show that this difference is due to defects in UNCD. (c) 2005 Elsevier B.V. All rights reserved.