We present thermal conductivity measurements of micromachined 500 nm thick silicon-nitride (Si–N) beams suspended between two Si–N islands, in the temperature range from 77 to 325 K. The measured thermal conductivity, k, of Si–N at high temperatures is in good agreement with previously measured values for Si–N grown by low-pressure chemical vapor deposition, but behaves much differently as temperature is lowered, showing a dependence more similar to polycrystalline materials. Preliminary structural characterization by x-ray diffraction suggests that the material is likely nano- or polycrystalline. The micromachined suspended platform structure is designed to allow highly accurate measurements of the thermal conductivity of deposited metallic, semiconducting, or insulating thin films. As a demonstration, we present measurements of a 200 nm thick sputtered molybdenum film. In the entire temperature range the measured thermal conductivity matches the prediction of the Wiedemann–Franz thermal conductivity determined from measured electrical conductivity.