Using electron paramagnetic resonance, we find that vacuum annealing at 200 °C leads to a significant reduction in the silicon dangling bond (Si-db) defect density in silicon nanoparticles (Si-NPs). The best improvement of the Si-db density by a factor of 10 is obtained when the vacuum annealing is combined with an etching step in hydrofluoric acid (HF), whereas HF etching alone only removes the Si-dbs at the Si/SiO2 interface. The reduction in the Si-db defect density is confirmed by photothermal deflection spectroscopy and photoconductivity measurements on thin Si-NPs films.