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In this work, we studied structural and magnetic properties of 18nm-sized Zn-substituted magnetite, 28nm-sized unsubstituted and 17nm-sized (Mn,Zn)-substituted iron oxide nanoparticles, synthesized by thermal decomposition method. Their features were examined by analyzing the X-ray diffraction (XRD) data, 57 Fe Mössbauer spectra and magnetization measurements by SQUID interferometer. The microstructure was inspected comparing different the size and strain broadening models incorporated into Fullprof software. In terms of crystallinity and size dispersion, applied synthesis protocol shows superiority over decomposition of iron oleate and the co-precipitation synthesis route. The saturation magnetization at T=5 K was found to be within the M S =91.2-98.6 Am 2 /kg range, while at 300 K M S of pure and Zn-substituted Fe 3 O 4 nanoparticles is 83.6 and 86.2 Am 2 /kg, respectively.Effective magnetic anisotropy constant K eff , estimated under slow measurements by SQUID, is below 20 kJ/m 3 in all three samples. Some preliminary measurements of the magnetic hyperthermia performance, expressed via Specific Absorption Rate (SAR) value showed that the best heating performances were displayed by 18nm-sized oleic acid-coated Zn 0.13 Fe 2.87 O 4 cubooctahedrons with SAR@425 W/g Fe at H 0 =20 kA/m and f=228 kHz.