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Magnetic nanoparticles (NPs) have attracted attention for their potential use in electronic devices and nano-bioengineering applications. Fe NPs made by thermal decomposition from an Fe(CO)x-Oleylamine (OlAm) reacted precursor show high saturation magnetization (Ms) (~ 140 emu/gnet at 300 K). However, the Ms never reaches to the bulk value (218 emu/g at 300 K) of iron due to the unique crystalline structure of the Fe NPs. Fe NPs coated with OlAm (OlAm-Fe NP) have an expanded α (b.c.c.) structure and ultra-fine grains. In this paper, we focus on the adsorption ability of the surfactant to improve Ms via control of phase and grain size. Fe NPs with high saturation magnetization were synthesized by thermal decomposition of Fe(CO)5 with some surfactants that had weak absorption ability, such as tribenzylamine, trioctylamine, and their mixture. Slow decomposition rate of Fe(CO)5 and large grain size were obtained by applying surfactants with weak adsorption ability. Especially, Fe NPs that were synthesized by mixed surfactants (mixed surfactant-Fe NPs) have polygonal shape with 6.7 nm in diameter. HRTEM and XRD results suggest that mixed surfactant-Fe NPs are single crystallines of α -Fe. Ms of mixed surfactant-Fe NPs is 194 and 183 emu/gnet at 5 K and 300 K, respectively. These results strongly indicate that single crystallization of α-Fe enhances the Ms of Fe NPs and also suggest that optimization of the surfactant adsorption ability is indispensable to promote the single crystallization of Fe NPs.