The spectrophotometric method and the electron microscope have been used to determine the distribution function of Ag@Fe3O4 nanoparticles by sizes and to measure their complex refractive index. These particles have been synthesized as acomponent of magnetically controlled drugs with antibacterial properties. The algorithm of processing the results of the experiment has been designed to measure the sizes of nanoparticles (1-100 nm).Key words: nanoparticle sizes measuring spectrum attenuation.