The iron oxide nanoparticles had been successfully synthesized via an additive-free hydrolysis process at 75 °C for 12 h. The product was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and N2 adsorption–desorption. The results of XRD and N2 adsorption–desorption demonstrated that the as-prepared product was mainly α-Fe2O3 with a large surface area of 164.1 m2 g−1. The TEM images illustrated that the as-prepared product was found to consist of a mixture of irregular spherical nanoparticles (a diameter of ∼50 nm) and nanowhiskers (a diameter of ∼50 nm and uneven length). The as-prepared product was used to investigate its promising applications in water treatment. Due to its small size and large surface area, the maximum adsorption capacities of Congo red and Cr(VI) have been determined using the Langmuir equation and found to reach up to 253.8 and 17.0 mg g−1, respectively. The facile synthesis method and the superior adsorption performance derived from the iron oxide nanoparticles display the potential applications for the removal of Congo red and Cr(VI) from aqueous solution.