Grain oriented 3 wt% silicon steel of 0.3 mm thickness was subjected to fracture toughness evaluation in the transition temperature regime at cross-head rates of 10-2, 10-4 and 10-6 ms-1. The observed values were compared to the Microscopically-Shielded Griffith Criterion (MGC) model [Acta metall. mater. 40, 2861 (1992)] which had previously predicted fracture toughness for plasticity induced cleavage of single crystals. From this, it is shown that the MGC model gives reasonable predictions of fracture toughness for cleavage at different loading rates and test temperatures. An analytical approximation of the computer simulation gives the interesting result that fracture toughness predominantly has a two-parameter dependence for a given material. It is directly proportional to an exponential function of the true surface energy and inversely proportional to an exponential function of the yield strength. It is also demonstrated that grain oriented 3 wt% silicon steel represents a useful experimental material for fundamental fracture toughness investigations.