Mn nanoparticles are prepared by arc discharge technique. MnO, alpha-Mn, beta-Mn, and gamma-Mn are detected by X-ray diffraction, while the presence of Mn3O4 and MnO2 is revealed by X-ray photoelectron spectroscopy. Transmission electron microscopy observations show that most of the Mn nanoparticles have irregular shapes, rough surfaces and a shell/core structure, with sizes ranging from several nanometers to 80 nm. The magnetic properties of the Mn nanoparticles are investigated between 2 and 350 K at magnetic fields up to 5 T. A magnetic transition occurring near 43 K is attributed to the formation of the ferrimagnetic Mn3O4. The coercivity of the Mn nanoparticles, arising mainly from Mn3O4, decreases linearly with increasing temperature below 40 K. Below the blocking temperature T-B approximate to 34 K, the hysteresis loops exhibit large coercivity (up to 500 kA/m), owing to finite size effects, and irreversibility in the loops is found up to 4 T, and magnetization is not saturated up to 5 T. The relationship between structure and the magnetic properties are discussed. (C) 2004 Elsevier Ltd. All rights reserved.