A new series of solid solutions Dy2-xGdxMo4O15 (x = 0.0-0.9) were prepared. These compounds all crystallize in monoclinic structure with space group P21/c. The lattice parameters a, b, c and unit cell volumes V increase almost linearly with increasing gadolinium content. The intrinsic thermal expansion coefficients of Dy2-xGdxMo4O15 (x = 0.0 and 0.25) were obtained in the temperature range of 25 to 500°C with high-temperature X-ray diffraction. The correlation between thermal expansion and crystal structure was discussed.