Spark Plasma Sintering (SPS) of nanostructured FeMo powder produces samples with satisfactory density, however the final grain size critically depends on the sintering temperature. Two groups (sets A and B) of samples have been examined by means of internal friction (IF) and dynamic modulus measurements carried out in successive test runs on the same samples to assess their structural stability. Set A and B had been sintered at 1113 and 1128 K and had an average grain size of 100 nm and 1 µm, respectively. TEM and XRD have been performed on the samples in as-prepared condition and after IF measurements cycles. The samples with smaller grains are more stable and substantially are not affected by grain coarsening which, on the contrary, occurs in those with grains of larger size. The heating up to 923 K during the tests diminishes dislocation density in both the groups. An anomalous trend of resonance frequency during the first test run in samples of set A has been ascribed to the formation of small cracks relaxing internal stresses.