The synthesis of iron oxide nanoparticle has been focused on recent research interest for the reason of their diverse potential applications. The synthesis of monodisperse nanoparticle is the most important, since the properties of these nanoparticles strongly depend on their dimensions. We have designed and prepared monodisperse, size & shape controllable, and highly crystalline nanoparticle. Metal-oleate complex consisting of metal precursors and sodium oleated is dissolved in water-ethanol-toluene solution, and the oleic acid is injected passivating the metal precursors. Also, we inject hydrazine to decompose the metal-oleic acid complex. Unfortunately most synthetic method produced few grams of nanoparticles. However we report the large scale synthetic method. We were able to synthesize about 20g of magnetite nanocrystals with low reaction temperature using hydrazine. Moreover, we control the particle size by varying the experimental conditions without size sorting process. The nanoparticles obtained were characterized by TEM, X-ray diffraction, X-ray absorption spectroscopy, X-ray magnetic circular dichroism, and Raman spectroscopy. Magnetite nanoparticle could be applied for MRI contrast agent, targeted drug delivery, magnetic hyperthermia for tumor treatment, biochip, and so on.