Room-temperature strength, fracture toughness, Young's modulus, and thermal-shock resistance were determined for 68 to 98% dense lithium orthosilicate (Li4SiO4) specimens. In the low-density regime, both strength and fracture thoughness were controlled by the density of the specimen. At high density, the strength depends on grain size. Young's modulus values ranged from 30 to 103 GPa at densities between 68 and 98% TD. A critical quenching temperature difference in the range of 150–170 °C was observed in thermal-shock tests of bar specimens. Steady-state creep tests indicated that 90% dense Li4SiO4 fractures at T ≤ 800 °C before reaching steady state and deforms plastically at 900 °C. It is more creep-resistant at 900 °C than Li2O, about equal to Li2ZrO3, and less than LiAlO2.