Nano-sized FeFe2O4 ferrite powder for local thermal coagulation therapy was synthesized by a reverse coprecipitation method. The crystal diameter for the samples from the reverse coprecipitation method was smaller than that from a normal coprecipitation method. The crystal diameter increased with an increase in the synthesized temperature for both methods. The maximum increasing temperature under the AC magnetic field was at around a 12 nm crystal diameter. The FeFe2O4 powder was oxidized to Fe2O3 by calcination at 600 degrees C in ambient air. The heating ability almost depended on the hysteresis loss value. Although the increased temperature under the AC magnetic field for the fresh FeFe2O4 sample was very close to that for the commercialized MgFe2O4 powder, it was gradually decreased for FeFe2O4 with time in ambient air. (c) 2005 Elsevier Ltd. All rights reserved.