The addition of small quantities of vanadium in structural steels produces a significant refinement in the final ferrite microstructure. There are two different mechanisms contributing to refinement: enhancement of grain boundary ferrite nucleation and intragranular nucleation. The contribution of each mechanism depends on the vanadium content and heat treatment of the steel. In this study the contribution of both refining mechanisms has been evaluated for two V-microalloyed steels subjected to different heat treatments. The results confirm that this refinement is based on the enhancement of ferrite nucleation through particle-stimulated nucleation mechanisms, while other aspects, as the influence of vanadium slowing down the austenite-ferrite transformation kinetics, seem to exert a minor effect.