Low-thermal-conductivity rare earth zirconates (Re2Zr2O7) have recently been identified as promising thermal barrier coating materials. We observed an order-disorder transition in the (Sm1-xYbx)(2)Zr2O7 series with the changing x value and investigated the thermal conductivity variation. Structural analysis by X-ray diffraction and Raman spectroscopy shows that the (Sm1-xYbx)(2)Zr2O7 series undergo a discontinuous phase transition from an ordered pyrochlore phase to a disordered fluorite one between the x=1/6 and x=1/3 compositions. Meanwhile, both of the sound velocity and Young's modulus reveal a dramatic reduction, indicating the lattice softening accompanying the order-disorder transition. The thermal conductivities of the (Sm1-xYbx)(2)Zr2O7 series are different from the conventional behavior of a simple alloying system and show a minimum thermal conductivity value at the transition composition (Sm2/3Yb1/3)(2)Zr2O7, which possibly arises from the enhanced phonon scattering due to the lattice softening.