In order to investigate the criterion of unstable crack propagation induced by singular stress field less than 10 nm, the fracture toughness is examined using micro-scale silicon (Si) single crystal specimens with a precrack. Critical value of stress intensity factor at fracture, K_<IC>, is evaluated to be 0.93〜1.09 MPa・m^<1/2> where the minimum size of singular stress field is 6.3 nm. This clearly means that the fracture mechanics concept based on the continuum mechanics keeps the validity even in the single digit nanometer scale.