Iron oxide magnetic nanoparticles (MNPs) are of interest in biomedicine and research owing to their moderate cytotoxicity and advanced properties, such as extensive surface-to-volume ratio and possibilities for tailoring their functionality through surface chemistry. To date, various approaches have been used for the synthesis of MNPs with controllable structural properties and various coatings to enhance their stability and functionality. This study describes a modified one-step method of coprecipitation in the presence of glycine allowing the production of particles with controllable size and in situ surface decoration. The effect of different glycine concentrations on the morphostructural and magnetic properties of iron oxide MNPs is studied. The particle size is reduced from 10.2 ± 0.3 to 7.2 ± 0.5 nm by increasing the glycine concentration from 0.06 up to 0.60 mol. The magnetic properties of obtained particles were tracked by SQUID magnetometry and Mössbauer spectroscopy. All samples of glycine capped iron oxide MNPs showed superparamagnetic behaviour at room temperature with maximal value of the saturation magnetization of 69 ± 4 Am²/kg. The results show the optimal concentration range of glycine which can be used in this method: a lower concertation than 0.15 mol does not affect the properties of obtained particles while higher concentrations than 0.3 mol lead to the reduction of magnetic properties (the saturation magnetisation reduces to 59 ± 3 Am²/kg when glycine concentration was 0.6 mol). The proposed economic and environment-friendly approach can be utilized to synthesise –NH₂ functionalised MNPs for biomedical or wastewater treatment.