The present paper describes the nature of crack tip plasticity in silicon crystals examined by high voltage electron microscopy (HVEM) and atomic force microscopy (AFM). Firstly, AFM images around a crack tip are presented, where the formation of fine slip bands with the step heights of one or two nanometers is demonstrated. Secondly, crack-tip dislocations observed by HVEM are exhibited, where it is emphasized that dislocation characterization is essential to consider the relief mechanism of crack-tip stress concentration.