Nanocrystalline trimanganese tetroxide (Mn3O4) has been successfully synthesized by a solvothermal reaction in which KMnO4 is reduced by CH3OH or CH3CH2OH under pressure. Alcohol also served as the solvent in the method. The reaction temperature can be as low as 80-100 degrees C. Transmission electron micrographs and X-ray powder diffraction patterns show that the average particle sizes are 9-15 nm. From the electron spin resonance (ESR) pattern, hyperfine structure (six absorption peaks of ESR) of Mn2+ ion is observed on the main line of Mn3+. The mechanism for the formation of crystalline Mn3O4 is proposed.