We report a facile one-step hydrothermal approach to the synthesis of iron oxide (Fe(3)O(4)) nanoparticles (NPs) with controllable diameters, narrow size distribution, and tunable magnetic properties. In this approach, the iron oxide NPs were fabricated by oxidation of FeCl(2)·4H(2)O in basic aqueous solution under an elevated temperature and pressure. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) studies reveal that the particles are highly crystalline and that the diameters of the particles can be tuned from 15 nm to 31 nm through the variation of the reaction conditions. The NPs exhibit high saturation magnetization in the range of 53.3 ~ 97.4 emu/g and their magnetic behavior can be either ferromagnetic or superparamagnetic depending on the particle size. A superconducting quantum interference device magnetorelaxometry (SQUID-MRX) study shows that the size of the NPs significantly affects the detection sensitivity. The investigated iron oxide NPs may find many potential biological applications in cancer diagnosis and treatment.