We report on the synthesis of size‐classified siliconnanocrystals by differential mobility classification of a polydisperse ultrafine siliconaerosol. Using the described technique, the average nanocrystal size can be tuned by simply adjusting an electric field. Samples of nonagglomerated siliconnanocrystals have been obtained and size control has been achieved within 15%–20% in the 2–10 nm size regime.