A model has been developed to predict the ferrite grain size distribution resulting after the austenite to ferrite transformation in slowly cooled low carbon steels. The model uses the austenite grain size distribution present before transformation as input and provides the size distribution of ferrite grains at any instant in the transformation range, whether the microstructure is partially either completely transformed. The model is based on empirical equations relating the mean austenite and ferrite grain sizes and log-normal shape grain size distributions. A validation of the model was carried out in the laboratory by torsion tests for Nb and Nb-V microalloyed steels.