Continuous hydrothermal synthesis of Fe2O3 nanoparticles was carried out at 673 K and 30 MPa. Two types of mixers, a conventional T-type micromixer and a central collision-type micromixer (CCM), were used for the synthesis. CCM was newly fabricated on the basis of the concept for preventing heterogeneous nucleation induced on the inner wall of the mixer. Residence time and Fe(NO3)(3) molality of the starting solution varied from 0.1 to 1.0 s and 0.05 to 0.50 mol/kg, respectively. Effects of the mixer structure, the residence time, and the Fe(NO3)(3) molality on crystal structure, Fe3+ conversion, average particle size, and size distribution were discussed. CCM effectively worked for producing smaller Fe2O3 nanoparticles with narrow and monomodal distribution.