The microwave absorption properties of a material depend largely on the dielectric properties of the material being heated. Therefore, the influences of temperature on the dielectric constant (ε'), loss factor (ε''), loss tangent (tan δ d) and penetration depth (D P) of steelmaking dust at frequencies of 1064 MHz and 2423 MHz were measured. Three steelmaking dust samples were studied. The effects of temperature on the dielectric properties of the samples were insignificant at temperatures below 600 °C. However, above this temperature, a rapid rise in the values of the dielectric properties of the samples was observed. Comparing the thermogravimetric analysis and differential scanning calorimetry (TGA-DSC) results and mass spectra (MS) of the dusts with their dielectric properties revealed that the changes in the dielectric values of the dusts were associated with the thermal decomposition of calcium carbonate and the release of CO/CO2 gases. Furthermore, the increase in the electrical conductivity of the samples at high temperature resulted in increased dielectric values. The behavior of the loss tangent of the samples with increasing temperature coincided with the behavior of the loss factor. The penetration depth decreased with an increase in temperature at both frequencies, while an increase in the dielectric properties caused a significant decrease in the penetration depth. The results indicated that steelmaking dusts have good microwave absorbing properties owing to their carbon and iron oxide contents.