A new series of rare earth compounds, Y2-xErxMo4O15 (x = 0.0-2.0), were synthesized and their structures were determined by powder X-ray diffraction (XRD). Rietveld analysis of XRD patterns reveals that the compounds of this series crystallize in monoclinic system with space group P21/c. The lattice parameters a, b, c, β and the unit cell volume V decrease linearly with increasing Er content. The thermal expansion properties of these compounds were studied under high-temperature XRD. Positive thermal expansions of compounds Y2-xErxMo4O15 are found to be anisotropic along the three crystallographic directions where a and c axes expand while b axis contracts in the whole temperature range. By analysis of data, the expansion of weak band Mo2-O14 with rising temperature should be responsible for positive thermal expansion.