Several rare earth solid solutions of Y1.9R0.1Mo4O15 (R=Sm, Nd, Dy) with interesting thermal expansion behavior were prepared and studied by X-ray and neutron diffraction. These solid solutions crystallize in a monoclinic space group P21/c. The lattice parameters a, b and c of Y1.9R0.1Mo4O15 increase with increasing of the ionic radius of substituted elementary. The high-temperature XRD data for Y1.9Sm0.1Mo4O15 show that there is an unusual low thermal expansion behavior in the 25~100°C range followed by a normal thermal expansion at higher temperature up to 600°C. Since no phase transition was observed, the temperature dependence of Mo2-O14 bond distance seems to be responsible for the anomalous thermal behavior.