In this paper, we report the crystal structure of the low temperature form of barium borate-a new type of SHG material. Ba3(B3O6)2 crystal belongs to trigonal system with space group C34-R3. There are 6 formula units in one hexagonal cell. The unit cell dimensions referring to hexagonal axes are a = b = 12.532?, c = 12.717?. Diffraction data were collected on Philips PW-1100 four-circle diffractometer. The number of independent reflections amounts to 693. The structure was determined by conventional three-dimensional Patterson and Fourier syntheses and refined by "block diagonal" matrix least-squres procedures to R = 0.046. This structure can be considered as a layer step type of lattice built up by Ba2+ and (B3O6)3- rings atterna-tively. The anion (B3O6)3- group is nearly planar with its plane perpendicular to the threefold axis which passes through the center of the ring. Each barium atom has seven oxygen neighbors. In comparison with the high temperature form of barium borate, it is shown that the acentric symmetry of the barium cations in this structure changes the electron density distribution of the inorganic conjugated boron-oxygen rings, thereby making principal contributions to the SHG effect.