The microstructural evolution and fire-resistant properties in the weld heat-affected zone (HAZ) of Mo and Mo + Nb-added fire-resistant steels were investigated. For this purpose, three Fe-0.1 wt%C-1.5 wt%Mn-0.1 wt%Si steels containing various Mo and Nb contents were prepared. HAZ samples were experimentally simulated using a Gleeble simulator at a welding heat input of 30 and 300 kJ/cm. The yield strength of the HAZ samples was higher than those of base steels at both room temperature and 600 celcius, whereas a greater decrease in the yield strength at 600 celcius compared to that at room temperature occurred in the HAZ samples than in the base steels, indicating that the fire-resistance deteriorated in the HAZs as compared to the base steels. This is due to the formation of hard phases such as bainite and martensite in the HAZs, i.e., bainite and martensite phase have very high yield strength with high dislocation density at room temperature, while their strengths decrease rapidly at high temperature due to a great annihilation and recovery of dislocations at high temperature. In addition, the fire-resistance of the HAZ improved as the heat input was increased. The alloying of Mo and Nb improved the fire-resistance of both the base steels and the HAZs. Finally, the changes in the microstructures of the base steels and the HAZs upon alloying and the heat input and corresponding effects on the fire-resistance were carefully explored and discussed through transmission electron microscopy analyses, atom probe tomography analyses, and calculations of continuous cooling transformation diagrams.