The accumulation of plastic waste in the world's oceans is a growing public concern, under the action of water and solar radiation, the macroscopic plastic objects break down into micro- and nano-sized particles. The amount of microplastics in natural waters is currently unknown because of the difficulties of their quantification in water. It is proposed to use the method of the preconcentration of micro- and nano- plastic particles in water using two main approaches, such as the addition of composite magnetic nanoseeds that form aggregates with detectable plastic particles, and the subsequent separation of these hetertoaggregates from water by magnetic separation. In order to concentrate polyethylene (PE, 20-100 µm) and polyethylene terephthalate (PET, 10-20µm) particles from water, the magnetic Fe-C-NH2 nanoseeds (10 nm) were added to the water, afterward, the magnetic sedimentation of the formed heteroaggregates in a gradient magnetic field was conducted. The effect of magnetic nanoparticles concentration in suspension, the time expose for the heteroaggregation on the efficiency magnetic preconcentration have been investigated.