The mechanical and thermal properties of single crystals of the type-I clathrate compounds Ba8Ga16Ge30 and Sr 8Ga16Ge30 have been investigated by measuring the elastic constants, coefficients of thermal expansion (CTE), and plastic deformation behavior in compression. The values of bulk moduli extrapolated to 0 K for Ba8Ga16Ge30 and Sr8Ga 16Ge30 are almost identical to each other, whereas the values of Young and shear moduli for Ba8Ga16Ge 30 are larger than those for Sr8Ga16Ge 30. The values of polycrystalline moduli for both Ba 8Ga16Ge30 and Sr8Ga 16Ge30 are all considerably smaller than those for pure germanium. The values of CTE for Ba8Ga16Ge30 and Sr8Ga16Ge30 are virtually identical with each other, but they are considerably larger than that of pure germanium. The feasibility of these two clathrate compounds as a thermoelectric material in terms of mechanical stability under possible thermal stresses is evaluated by calculating thermal stresses that are expected to develop within these compounds when used as a thermoelectric material in thermoelectric devises.