Magnetic nanoparticles as effective heat-mediator agents in hyperthermia treatment might be able to produce required heat for killing cancerous cells. Superparamagnetic iron oxide nanoparticles (〖Fe〗_3 O_4) with different sizes in the range of 5-19 nm were successfully fabricated by co-precipitation method. In order to control particle size and also stabilize nanoparticles in physiologic conditions, the nanoparticles were covered by a biocompatible layer of dextran through in situ and semi-two-step methods. The experimental results confirms the superparamagnetic behavior of all samples. The findings also show that the smallest nanoparticles (with average size of 5 nm) are obtained by in situ method in which dextran acts as surfactant during precipitation of nanoparticles. Also, the best stability is devoted to the fluids containing these particles which are highly stable at physiological pH for several months. Furthermore, in vitro cytotoxicity measurements, performed on L929 cell line (mouse fibroblast), reveal that the presence of biocompatible and protective layer of dextran significantly increase the viability of dextran coated nanoparticles. Heating efficiency of magnetic fluids was evaluated by calorimetric measurements under the high-frequency magnetic field. The experimental results show that the specific absorption rate (SAR) of nanoparticles, defined as the heat generated per gram of magnetic nanoparticles at unit of time, is depended on the size and concentration of nanoparticles as well as field strength and frequency. However, the size suitability is a necessary and sufficient condition for heat generation by superparamagnetic magnetic nanoparticles, so that the nanoparticles with smaller size cannot produce any appreciable heat under a safe alternating magnetic field used in biomedical applications. In addition, the larger magnetic nanoparticles (with average size of 19 nm) have the optimized SAR and consequently can be used as efficient heating mediat
