The resistance to temper softening in low carbon martensite with its underlying origin, by microalloying of strong carbide-forming alloying elements (V, Nb and Ti) to an Fe-0.1C-1.5Mn-0.05Si (mass%) alloy, was investigated in this study. With similar hardness in as-quenched condition in all the alloys used, the hardness of tempered martensite is increased by V, Nb and Ti additions, particularly after treatment at higher temperature with longer time. The increment in hardness becomes larger by more amount of V addition, while with almost the same amount of microalloying additions, Nb and Ti provide larger strengthening than that of V. Atom probe measurements have revealed that a high density of nano-sized alloy carbides are formed in those alloys with V, Nb and Ti additions at 923 K, where large secondary hardening was observed. At 723 K, where there is some resistance to temper softening, however, almost no precipitation of V, Nb and Ti can be detected. The X-ray line profile analysis of the tempered alloys implies that the reduction in dislocation density during tempering is strongly retarded by V, Nb and Ti additions. This should be the major reason for their resistance to temper softening at relatively lower temperature, even without nano-precipitation of alloy carbides.