Nanoparticles of iron oxides have been prepared by the levitational gas condensation (LGC) method, and their structural and magnetic properties were studied by XRD, TEM and Mossbauer spectroscopy. Fe clusters were evaporated from a surface of the levitated liquid Fe droplet and then condensed into nanoparticles of iron oxide with particle size of 14 to 30 nm in a chamber filled with mixtures of Ar and $O_2$ gases. It was found that the phase transition from both $gamma$-$Fe_2O_3$ and $alpha$-Fe to $Fe_3O_4$, which was evaluated from the results of Mossbauer spectra, strongly depended on the $O_2$ flow rate. As a result, $gamma$-$Fe_2O_3$ was synthesized under the $O_2$ flow rate of 0.1$leq$$Vo_2$(Vmin)$leq$0.15, whereas $Fe_3O_4$ was synthesized under the $O_2$, flow rate of 0.15$leq$$Vo_2$(Vmin)$leq$0.2.