Abstract: We report here the development of a seed-mediated growth method for the synthesis of iron oxide nanoparticles with tunable size distribution and magnetic properties. We have investigated the size distribution of seed as well as iron oxide nanoparticles during the growth process using transmission electron microscopy (TEM). It has been observed that the distribution of size gradually becomes narrow with time via the intraparticle ripening process and Oswald ripening process. The magnetization measurements were performed using a superconducting quantum interference device (SQUID) from room temperature to 5 K to determine the effect of size distribution on the blocking temperature (Tb) and splitting temperature (Tsplitting). The thermogravimetric analyses exhibited the size-dependent weight loss of the magnetic nanoparticles. The in vitro cytotoxicity tests were also performed to determine the cell viability as a function of size and concentration of the magnetic nanoparticles.