Hot torsion testing was used to determine the effect of low carbon contents on the activation energy for plastic deformation in structural steels. It was found that at carbon levels lower than 0.15 wt.%, the plastic behavior is a consequence of the deformation conditions, where strain mechanisms are governed by dislocations or by diffusion. The strain rate compensated parameter (Zener-Hollomon parameter Z) has been used to correlate the maximum stress. The activation energy for deformation at constant temperature, has been evaluated considering the dominating mechanism. Experimental results show that each strain mechanism are characterized by their corresponding activation energy. In steels with very low carbon content, the activation energy rises when the carbon content increases at high Z values, due to the glide and interactions of dislocations. The diffusional flow, however, at low Z values is dominant causing the decrease of the activation energy with increase in carbon content. This explains the apparent discrepancy in the flow stress behavior at low carbon contents observed in the literature.