Iron oxide nanoparticles (FeNPs) were synthesized by green method using black tea (BT) extract as a reducing agent at room temperature. The effect of polyphenol content on the formation of FeNPs was investigated. The morphology of FeNPs could be controlled by changing the amount of BT extract and iron ions. The results showed that size and shape of the nanoparticles depended on the amount of polyphenol content of the mixture. Synthesized iron oxide nanoparticles were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy.