Size-controlled spinel iron oxide (SIO) nanoparticle/organic hybrid was synthesized in situ from iron (III) allylacetylacetonate (IAA) at around 80 °C. The formation of SIO particles chemically bound with organics was confirmed by infrared and x-ray photoelectron spectroscopy. The sizes of SIO nanoparticles in the hybrids were monodispersed and ranged from 7 to 23 nm under controlled hydrolysis conditions. The hybrid including SIO particles of 7.3 nm was superparamagnetic, whereas those dispersed with particles above 11 nm were ferrimagnetic. The specific absorption rate (SAR) value was dependent upon the magnetic properties of the hybrid at 100 Oe. The SAR was 15.2 W g−1 in a 230 kHz alternating magnetic field and 100 Oe when the crystallite size of SIO particle in the hybrid was 16 nm. The temperatures of agars dispersed with hybrid powders of 5 and 8 mg ml−1 reached the optimum temperature (42 °C) for 17 and 8 min, respectively. The increase in temperature was controlled in terms of the strength of magnetic field. The simulation of heat transfer in the agar phantom model revealed that the suitable temperature distribution for therapy was attained from 15 to 20 min at 230 kHz and 100 Oe.