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The adiabatic shear sensitivity of ultra-high-strength steels is closely related to their thermal conductivity. Therefore, it is essential to investigate the effects of alloying elements on the thermal conductivity of ultra-high-strength steel. In this study, the variation in the scattering behavior of electrons with respect to temperature and the mechanism of three-phonon scattering were considered for obtaining the contributions of electrons and phonons, respectively, to the thermal conductivity of alloys while solving the Boltzmann transport equation. By predicting the effect of ten alloying elements on the electronic thermal conductivity (<i>κ_e</i>), it was found that, at 1200 K, the doping of iron with Ni and Cr endowed iron with <i>κ_e</i> values of 24.9 and 25.7 W/m K, respectively. In addition, the prediction for the lattice thermal conductivity (<i>κ_L</i>), which was performed without considering point defect scattering, indicated that elements such as Al, Co, Mn, Mo, V, and Cr demonstrate a positive effect on the lattice thermal conductivity, with values of 3.6, 3.7, 3.0, 3.1, 3.9, and 3.8 W/m K, respectively. The contribution of <i>κ_L</i> is only 5–15% of the total thermal conductivity (<i>κ</i>_total). The alloying elements exhibited a similar effect on <i>κ</i>_total and <i>κ_e</i>. Δ<i>κ_i</i>; the change in thermal conductivity with respect to <i>κ</i>₀ owing to the alloying element i was evaluated according to the total thermal conductivity. These values were used to understand the effect of the concentration of alloying elements on the thermal conductivity of iron. The Δ<i>κ_i</i> values of Ni, Co, and W were 6.44, 6.80, and 6.06, respectively, indicating a reduction in the thermal conductivity of iron. This paper provides theoretical guidance for the design of ultra-high-strength steels with a high thermal conductivity.