Platinum nanoparticles were produced by laser ablation of a platinum metal plate in an aqueous solution of sodium dodecyl sulfate (SDS). The absorption spectrum of the platinum nanoparticles was essentially the same as that of platinum nanoparticles chemically prepared in a solution. The size distribution of the nanoparticles thus produced was measured to be in the range of 1-7 nm in diameter by an electron microscope. The abundance of the nanoparticles changes with the concentration of SDS in the solution. Stable platinum nanoparticles were found to be produced by this method even in pure water.