Manasa Jillella, Dattatri K. Nagesha, Rajiv Kumar, Codi Gharagouzloo and Srinivas Sridhar æMagnetic iron oxide nanoparticles are increasingly being utilized as contrast enhancing agents in magnetic resonance imaging (MRI), magnetically targeted drug delivery agent and in magnetic hyperthermia. Recent developments have shown that size and size distribution of nanoparticles play a very critical role in the biomedical application of these magnetic nanoparticles. Monodisperse iron oxide nanoparticles synthesized by high temperature thermal decomposition method æare uniform in size and the size tunable by changing the experimental conditions. In this work, we have carried out a systematic study on the effect of particle size of iron oxide nanoparticles on its MRI contrast and magnetic hyperthermia. In the first step, nanoparticles of various sizes were synthesized and then coated with DSPE-PEG and were characterized for their size and morphology using DLS and TEM. In the second step, hydrophobic drug doxorubicin (DOX) was loaded with the core of the different size iron oxide nanoparticles. We further studied the effect of external magnetic field on its ability to trigger the release of drugs. The use of external oscillating magnetic field ensures the localized release of drugs only at the site of accumulation thus minimizing damage to adjacent areas. In vitro studies were carried out to study the efficacy of DOX in HeLa cervical cancer cell line. Results from the size dependent MRI contrast from phantoms and cancer cells incubated with nanoparticles and the effect of magnetic field on triggered release of drugs will be presented.