Crystal structures and thermal expansion properties of Dy 2−x Ln x Mo 4 O 15 (Ln=Ho (x=0.0–2.0) and Sm (x=0.0–0.4)) have been investigated by means of X-ray diffraction (XRD). Dy 2−x Ln x Mo 4 O 15 (Ln=Ho and Sm) crystallizes in monoclinic lattice with space group P2 1 /c in the 25–500°C temperature range. High-temperature XRD result reveals an anisotropic expansion in the lattice of Dy 2−x Ln x Mo 4 O 15 with the expansion of a- and c-axes and contraction of b-axis. Despite the shrinkage of b-axis, the volume thermal expansion coefficient of Dy 2−x Ln x Mo 4 O 15 is positive. The thermal expansion behavior of Dy 2−x Ln x Mo 4 O 15 is basically related to the temperature-induced expansion of bond Dy(Ln)–O and Mo(2)–O14.