Nanoparticles of An, Ag, Pt and Cu (unimetal nanoparticles) and mixtures of two of them (bimetal nanoparticles) are prepared through photocatalytic reduction of the corresponding metal ions in H2OEtOH solution in the presence of ultrafine TiO2 particles which are made so as to disperse stably in various organic solvents. The resultant nanoparticle solutions show intense colors owing to surface plasmon absorption of the nanoparticles and have high colloidal stability. An unimetal nanoparticles isolated from the solution redisperse stably in organic solvents and their dispersibility increases when they are treated with THF. Ag-Au and Pt-Au bimetal nanoparticles are prepared through successive photoreduction that Au ion first and then Ag or Pt ion is photoreduced. The bimetal nanoparticles are found to be not alloyed but to be mixtures of unimetal nanoparticles. The mechanism for the formation and the colloidal stabilization of the nanoparticles in the presence of the ultrafine TiO2 particles are suggested to be different from that for reaction systems using bulk TiO2.