The strengthening mechanisms which are operative in bainite are very well known: small bainite packet, small width of the laths, dislocation density and size and number of carbide particles (Fe 3 C), among others. Bainite packet size has been traditionally c onsidered as the value measured by optical microscopy (OM), as electron back scattered diffraction (EBSD) technique is relatively recent. In a V - microalloyed steel with bainitic microstructure of C=0.38% , V=0.12% and N= 0.0214% the average length and width of ferrite laths and of cementite carbides were measured. On the other hand, the bainite packet size was measured by OM and EBSD with a misorientation of 15º . These values of the microstructural units have been taken in account to calculate the effective surface energy  p given by Griffith’s model for cleavage fracture. It was concluded that bainite packet size determined by EBSD with a misorientation angle crit erion of 15º was the microstructural parameter that controls cleavage crack propagation. Given the relationship between the average unit crack path (UCP) and the bainite packet size, it was concluded that the effective surface energy of cleavage fracture (  p ) would be between 71.6 and 82.6 J m - 2