The nanocrystalline orthorhombic LiMnO2 was successfully synthesized with the raw materials of MnO2, Mn (CH3COO)2 and LiOH by hydrothermal synthesis method, which has the properties of small size, less stacking faults and single phase. High resolution electron microscopy images show that along the b axis of the orthogonal LiMnO2, MnO6 and LiO6 octahedron arrange alternately and regularly. The orthorhombic LiMnO2 is proved to be p-type semiconductor by the Hall test. XPS tests indicate that the trivalent Mn in the o-LiMnO2 is in the high-spin state. And the magnetic study shows that there is reentrant spin glass behavior in o-LiMnO2. The contrast study of Raman spectroscopy and magnetic susceptibility shows that the characteristic mode softening of structural phase transition corresponds to the magnetic phase transition temperature, indicating a possible interaction between phonons and spin.