The thermal conductivity (k) of silicon thin films can be reduced by additional phonon scattering at boundaries of the thin film. Though the lower k makes thermal management in electronics devices challenging, it is promising for an enhancement of the power factor (for thermoelectric devices) in silicon nanostructures, which has been demonstrated in recent experimental and theoretical studies. Beyond nanostructuring, mechanical strain impacts both the electron and phonon transport in nanostructures. This work focuses on using strain engineering to reduce thermal conductivity in order to further improve the thermoelectric figure of merit (ZT) in sub-40-nm silicon nanofilms. While past simulations showed an impact of strain on thermal transport in semiconductor films, there is not yet a conclusion on its impact on ZT due to the conflicting simulation results. Thus, here, we systematically measure the size-, strain-, and temperature- dependent thermal conductivity to elucidate the strain-dependent phonon transport in strained silicon nanostructures. In addition to the impact of strain on thermal transport in silicon nanofilms, we evaluate the potential impact on ZT.