The magnetic properties of nano magnetic particles depend substantially on processes of formation of nanoparticles, as well on polymer involved in its formation. We studied the effect of polymer as encapsulating agent on synthesis, magnetization.     Magnetite (Fe3O4) nanoparticles were prepared by chemical coprecipitation from a ferrous/ferric mixed salt-solution in the presence of PVP used as a stabilizer and without it. Encapsulated magnetite nanoparticles (Fe3O4) were prepared according to the method of chemical coprecipitation in the presence of poly-N-vinylpyrrolidone (PVP) (MW: 8000).The presence of PVP gives possibility of formation encapsulated nanoparticles.   The resulting magnetite nanoparticles were characterized by X-ray powder diffraction (XRD) analysis, scanning electron microscopy (SEM) and magnetization measured by VSM (vibrating sample magnetometer). Nanoparticles magnetization curves as function of external magnetic field intensity at low temperature 5 K and room temperature 300 K are shown in (Fig.1).     Fig1. Nanoparticles magnetization  a) T = 5K,  b) T = 300K.   At 5K, all samples exhibit the hysteresis phenomenon with low values of coercive force. At room temperature, the magnetization curves show vice-versa the absence of hysteresis phenomenon and the values of coercive force decrease to zero, which indicates their superparamagnetic properties. It means, that the dimension of obtained nanoparticles do not exceed the critical value for superparamagnetism, which equals to 29 nm and well correspond to the results electronic microscopy measurements and XRD. The maximum value of magnetization is 58 emu/g at 5 K and 45 emu/g at 300 K corresponding to the sample of 14 nm size.