The structure-activity relationship is crucial in catalytic performance and material design but still largely obscure due to the complexity of heterogeneous catalytic systems. CO activation occurs widely in Fischer-Tropsch reactions and pyrometallurgy, and it is a key to understanding carburization. Here, we investigate the structure-activity relationship in Fe nanoparticles by reactive molecular dynamics simulations. We focus on two activities, the adsorption and dissociation of CO, and four structural characteristics, morphologies, sizes, defects, and heteroatoms. The results show that CO adsorption and dissociation varies with the change of nanoparticles. Line dislocation and vacancies can strikingly boost CO dissociation, suggesting an effective way to tune the CO dissociation rate. Further analysis shows that the Eley-Rideal mechanism possibly works in the early periods, followed by the Langmuir-Hinshelwood mechanism in the later periods for CO2 formation. Our results shed light on the mechanism and possible optimization of the carburization of iron. (C) 2019 Published by Elsevier Inc.