Thermal conductivity is an important material parameter of silicon when studying the performance and reliability of devices or for guiding circuit design when considering heat dissipation, especially when the self-heating effect becomes prominent in ultra-scaled MOSFETs. The cross-plane thermal conductivity of a thin silicon film is lacking due to the difficulty in sensing high thermal conductivity in the vertical direction. In this paper, a feasible method that utilizes an ultra-fast electrical pulse within 20 μs combined with the hot strip technique is adopted. To the best of our knowledge, this is the first work that shows how to extract the cross-plane thermal conductivity of sub-50 nm (30 nm, 17 nm, and 10 nm) silicon films on buried oxide. The ratio of the extracted cross-plane thermal conductivity of the silicon films over the bulk value is only about 6.9％, 4.3％, and 3.8％at 300 K, respectively. As the thickness of the films is smaller than the phonon mean free path, the classical heat transport theory fails to predict the heat dissipation in nanoscale transistors. Thus, in this study, a ballistic model, derived from the heat transport equation based on extended-irreversible-hydrodynamics (EIT), is used for further investigation, and the simulation results exhibit good consistence with the experimental data. The extracted effective thermal data could provide a good reference for precise device simulations and thermoelectric applications.