Si nanocrystals (Si-ncs) were prepared from n-type Si (100) wafers by using high-current pulsed electron beam irradiation in vacuum. High density (9.7 × 10¹⁴ cm⁻²) ultrafine Si-ncs (∼3 nm) embedded in amorphous Si layers (∼60 nm) exhibit blue photoluminescence emission at room temperature which can be attributed to the quantum confinement size effect. The pulse number is a key parameter which influences the photoluminescence intensity. Possible formation mechanisms of Si-ncs are discussed.