The present study is concerned with the investigation of the micro mechanisms of micro defect nucleation in bainitic steels in order to provide an enhanced basis for probabilistic cleavage models. By a micro mechanical modelling of the cleavage initiation process the effects and the interactions of the relevant parameters can be identified. For this purpose Representative Volume Elements (RVE) of the micro structure are utilised, accounting for both, the grain structure as well as the brittle particles at the grain boundaries. The RVE's are loaded based on the local mechanical field quantities determined numerically at the cleavage origins of different fracture mechanics specimens. (© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)