We report on the synthesis of Mn3O4 nanoparticles via a polyethyleneglycol (PEG)-assisted hydrothermal route. Crystalline phase was identified as Mn3O4. The crystallite size from X-ray Diffraction (XRD) pattern and particle size from transmission electron microscopy (TEM) micrographs were calculated as 23 +/- 1 nm and 24.5 +/- 0.5 nm, respectively. Nanoparticles (NPs) exhibit mainly tetragonal and spherical morphologies. Magnetic characteristics of Mn3O4 NPs were evaluated by electron spin resonance (ESR) in the temperature range of 24-294 K and the Curie temperature was observed as 43 K. Also the magnetic phases that occur in nanosize Mn3O4 are detected below Tc by this method. The room-temperature paramagnetic characteristic are verified by vibrating sample magnetometry (VSM).