Abstract Magnetic nanoparticles (Fe3O4) coated with polyethylene glycol (PEG), (Fe3O4/PEG), were synthesized by chemical co-precipitation of Fe2+/Fe3+ salts by aqueous ammonia in PEG solution. Radiation polymerization of 2-hydroxyethyl methacrylate (HEMA) monomer solution onto Fe3O4/PEG was performed at different doses to synthesize (Fe3O4/PEG)-pHEMA, namely FPH, nanocomposites. Properties of FPH nanocomposites were characterized by FT-IR, XRD, SEM, TEM, DLS, ESR and TGA techniques. The XRD of FPH nanocomposites showed all the peaks of Fe3O4 nanoparticles. SEM was used to assess the surface morphology of FPH. TEM showed that the average diameter of FPH nanocomposites was in the range of 9–40 nm. The thermal stability of FPH nanocomposites was higher than that of Fe3O4 and Fe3O4/PEG. Radio-labeling of (Fe3O4/PEG)-pHEMA nanocomposite irradiated at 10 kGy (FPH10) with 99mTc was performed using stannous chloride as reducing agent. Factors affecting the labeling yield (%) such as the substrate amount, the amount of reducing agent, the pH of reaction medium, the reaction time and the reaction temperature were investigated. The maximum labeling yield was 93% using 0.25 mg of FPH10 at pH 6 and 20 min reaction time. The biodistribution study of 99mTc-FPH10 was examined on two groups of ascites and solid tumor bearing mice. The biodistribution results referred that 99mTc-FPH10 was rapidly uptake in tumor sites ascites or solid tumors. The results indicated that FPH nanocomposites could be potentially used for tumor imaging and therapy.