Iron oxide nanoparticles with controlled morphology and magnetic properties have been an interesting area of research due to their unique magnetic, electronic, optical and mechanical properties. The one-pot synthesis of such particles serves as the objective of this paper. A family of acicular oxide nanoparticles was prepared by heating 50 vol% ethanol solution of 28 mM iron (III) chloride and 0.3 mM polyethylene glycol (20 kDa) in the presence of hydrazine monohydrate. The precipitate that was formed prior to the heat treatment step and the final reaction products were collected and analyzed by XRD and TEM in order to understand the particle formation mechanism. The results showed that by controlling the amount and the addition rate of hydrazine into the solution, acicular iron oxide of akaganeite, goethite, or magnetite phase was obtained at 120°C. Increasing the solution pH to greater than 13 and varying the heat treatment time was found to result in the formation of magnetite nanorod and nanowire. In conclusion, iron oxide nanoparticles with different phase, morphology, and purity had been prepared from a solution of iron (III) chloride and polyethylene glycol by varying the amount and addition rate of hydrazine monohydrate to the solution, the solution pH and the heating time and temperature.