in order to clarify the formation mechanism of nano-size particles by laser ablation of metal targets in water, laser ablation at the target-solvent interface was investigated by means of microsecond-resolved shadowgraphy for Ag, An, and Si targets in water. Two types of ablation ejection were observed at the target-solvent interface. One is a straight jet which was observed below 100 mus after a laser pulse. The other is a bubble which was developed below 1 mus and expanded up to a few millimeters in diameter at 200 mus and subsequently collapsed. We propose that the straight jet is ascribable to ejection of clusters and droplets of metal which must be a source of nano-size colloidal particles. On the other hand, the bubble is assigned to water-confined plasma gas. Nano-particles must be also generated by the plasma ablation. (C) 2004 Elsevier B.V. All rights reserved.