It is well known that microalloying of Nb into low alloyed steels improves the toughness by refining the grain structure during rolling in the austenite regime. However, less effort has been devoted to the effect of Nb on carbide precipitation in the heat treatment processes after rolling. In the present study, the carbide precipitation was investigated in quenched and tempered (Q&T) high strength Cr-Mo steels. The results indicated that microalloying of Nb significantly improves the toughness of the Q&T steels. The effect is attributed to the alteration in precipitation of Mo-rich KSI-carbides caused by the presence of Nb. KSI-carbides preferentially precipitate on grain boundaries after tempering in the Nb-free steel. Enrichment of Mo in NbC carbides after quenching are observed by atom probe tomography (APT) in the Nb-microalloyed steel. Such precipitates act as new preferential sites for precipitation of KSI-carbides in the subsequent tempering process, resulting in reduced precipitation of KSI-carbides on grain boundaries. Based on the experimental findings in the present study, a synergistic scenario which rationalizes the findings addressing the role of Nb and Mo in improving the toughness is proposed.