The atomic structures of crack walls due to cleavage fracture in silicon nitride (Si3N4) and magnesium oxide (MgO) have been investigated by in situ straining transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM) at room temperature. In the case of Si3N4, the crack walls on the (1 1 0 0) plane were atomically flat, which indicates that the crack propagated along a particular crystal plane without deflection. On the other hand, the cleaved crack walls in MgO were not atomically flat but contained a number of square-shaped steps with a few (0 1 0) atomic layers in height. Thus, it can be said that the cleavage crack was deflected at the atomic level during its rapid propagation. The origin of their step structures is discussed using results from molecular dynamics (MD) simulations. (C) 2004 Elsevier B.V. All rights reserved.