A novel borate compound YBa3B9O18 has crystallized in a melt of BaYB9O16. Single-crystal X-ray diffraction measurements reveal that YBa3B9O18 adopts a hexagonal space group P(3 m with cell parameters of a = 7.1761(6) Angstrom and c = 16.9657(6) Angstrom. The structure is made up of the planar B3O6 groups parallel to each other along the (001) direction, regular YO6 octahedra, and irregular BaO6 and BaO9 polyhedra to form an analogue structure of beta-BaB2O4. A series of isostructural borate compounds RBa3B9O18 (R = Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb) were prepared by powder solid-state reactions. The DTA and TGA curves of YBaB9O16 show an obvious weight loss at about 955 degreesC associated with a decomposition into YBO3, B2O3, and YBa3B9O18 due to its incongruent melting behavior. The DTA and TGA curves of YBa3B9O18 show that it is chemically stable and a congruent melting compound. A comparison of the structures of YBa3B9O18 and beta-BaB2O4 is presented.