The condition for the formation of single phase of BaNdMnO4 was studied. c/a ratio decreases with oxygen nonstoichiometry because of liberation from the Jahn-Teller effect of trivalent Mn ion. The electrical conductivity shows a semiconducting behavior with the activation energy of 0.2 eV. BaNdMnO4 has paramagnetism and high spin state of Mn+3. The electron diffraction patterns of BaNdMnO4 reveal the existence of superstructure corresponding to 100 and 010 spots in this body centered tetragonal phase, from which a rotation of oxygen octahedra about c-axis is deduced. The high resolution electron microscopy (TEM) shows that thin layered BaNd2Mn2O7 phase is also included in the matrix of BaNdMnO4 phase as a kind of defect structure.