We have measured the thermal conductivities of a 53-nm-thick and a 64-nm-thick tin dioxide (SnO2) nanobelt using a microfabricated device in the temperature range of 80–350 K. The thermal conductivities of the nanobelts were found to be significantly lower than the bulk values, and agree with our calculation results using a full dispersion transmission function approach. Comparison between measurements and calculation suggests that phonon–boundary scattering is the primary effect determining the thermal conductivities.