The unstable cracking behavior in nanoscale single crystal silicon, including initiation, unstable propagation and arrest, is experimentally observed by using a nanoscale trapezoidal-double-cantilever-beam method. A well-controlled multi-step cracking experiment is designed for accurately estimating both the fracture toughness K Ic and the arrest toughness K Ia. The experimental results show that the unstable cracking within a short range of hundreds of nanometers leads to an apparent decrease from K Ic to K Ia, ie, K Ia< K Ic, and produces surprisingly clean crack surfaces with negligible energy dissipation. The specific surface energy of (0 1 1) cleavage plane in nanoscale single crystal silicon is accurately evaluated as γ= 1.83 J/m 2. These results provide a fundamental understanding of the unstable cracking behavior in a brittle material at the nanoscale.