The thermal conductivity (κ $\kappa $) and thermal diffusivity (D $D$) of talc have been measured over a range of temperature (298–1,373 K) and pressure (0.5–3.0 GPa) conditions using the transient plane‐source method. The results show that both the thermal conductivity and thermal diffusivity are dependent upon the prevailing temperature and pressure conditions to a certain extent. As the temperature and pressure increase, the thermal diffusivity monotonically decreases, while the thermal conductivity initially decreases between 298 and 973 K and then increases from 973 to 1,173 K. At low temperatures, phonon scattering is the dominant mechanism for heat transfer; at higher temperatures, photon radiation and dehydration become more prevalent. At temperatures greater than 1,173 K, the thermal conductivity decreases significantly due to aqueous liquid accumulation. Talc may be the cause of the high geothermal gradient in the hot subduction zone. Plain Language Summary: Subducting slab has a relatively low temperature compared to the surrounding mantle. Thermal conductivity and thermal diffusivity are important thermophysical properties that yield valuable information about the thermal structure of subduction zones and the thermal evolution of the Earth's interior. Talc plays a vital role in subduction zones because it carries a lot of water. There is currently little information available about the thermal and physical properties of talc during or after dehydration. We reported new results using pulse‐heating method and found that both the thermal conductivity and thermal diffusivity are dependent upon the prevailing temperature and pressure conditions to a certain extent. We also did the numerical simulations on the thermal state of subducting slabs with our experimental data, and found that the presence of talc in a subduction zone could effectively improve the geothermal gradient of the subducting slab. We believe that this result has an important implication for the study of the thermal structure of subduction zones. Key Points: A novel pulse heater was designed for use in high pressures experiments measuring the thermal conductivity and thermal diffusivityThermal conductivity and thermal diffusivity of talc were measured before and after dehydration at the temperature and pressure of 298–1,373 K and 0.5–3.0 GPaTalc may contribute to a high geothermal gradient in the subduction zone [ABSTRACT FROM AUTHOR]