The photochemical synthesis of small metal nanoparticles (Pd, Pt, Rh, Ir) was transmitted into a micro segmented flow process to control all aspects of the reaction. Solutions of noble metal salt, a photoinitiator, and a stabilizer were combined in a cross mixer and transferred in micro fluid segments. The start and termination of the photoinitiation, and thus the reduction of the noble metal salt, are defined by the entrance and the exit of fluid segments into the UV-ray of the irradiation source. The time of irradiation can be controlled by the flow rate. As a result, noble metal nanoparticles with comparatively high size homogeneity were obtained. The small nanoparticles are suitable as seed particles for the formation of larger particles with adjustable diameter.