In the ternary system of rare earth-cobalt-boron, RCo_3B_2 compounds (R=rare earth elements) were prepared by arc-melting methods. Their crystal structure was investigated by X-ray diffraction methods. These ternary borides, RCo_3B_2, crystallize in a hexagonal lattice. The lattice parameters are a=5.020±0.002A and c=3.027±0.002 A for YCo_3B_2 and a=5.066±0.003 A and c=3.022±0.002 A for GdCo_3B_2. The good agreement between the X-ray diffraction intensities observed and calculated shows that the ternary borides, YCo_3B_2 and GdCo_3B_2, crystallize in the CaZn_5-type structure. The space group and atomic positions are as follows : P6/mmm (D^1_), 1R in 1(a), 3Co in 3(g), and 2B in 2(c). It can be seen from these results that the ternary borides, RCo_3B_2, have a superstructure in which two Co atoms in the 2(c) site of intermetallic compounds, RCo_5, with the CaZn_5-type structure are replaced by two B atoms. The B atoms in this structure are situated at the center of a trigonal prism formed by six Co atoms. The interatomic distances between B and the six Co atoms are 2.11-2.10 A and are fairly constant, although the radii of the rare earth atoms are changed according to the lanthanide contraction. We have also found RCo_3B_2 compounds to be isostructural with YCo_3B_2 and GdCo_3B_2, where R=Ce, Sm, Tb, Dy, Ho, and Er. Efforts to prepare LaCo_3B_2, PrCo_3B_2, and NdCo_3B_2 by arc-melting were unsuccessful
