We reported a simple solvothermal route in phase-controllable synthesis of iron oxide nanoparticles by using polyols as solvent. Magnetite and hematite are selectively synthesized while Fe(NO3)3.9H2O is used as single iron source and without any additives. While ethylene glycol, 1,2propanediol, 1,3-butanediol, or glycerol is used as solvents, magnetite nanoparticles are obtained, and the average particle sizes vary from 10 to 40 nm. While 1,4-butanediol or 1,5-pentanediol is used as solvents, hematite nanoparticles are obtained. Our results suggest that the polyols with neighboring hydroxyl groups is favorable for the formation of magnetite nanoparticles.