Crack propagation mechanisms in a sintered silicon nitride containing various oxide additives (ceria, magnesia, zirconia and strontium oxide) were studied as a function of initial flaw size, temperature, applied stress and time. Surface cracks of controlled size were introduced using the microhardness indentation-induced-flaw technique. At 20° C, the fracture stress was found to depend on initial crack size according to the Griffith relationship and extrapolation of the data indicated that processing flaws of 20 to 35 μ were strength-controlling. The flexural strength was found to be independent of temperature from 20 to 800° C and the mode of crack propagation was primarily transgranular. At temperatures above 800° C the flexural strength decreased significantly, due to viscous flow of the glassy phase present in the material and resulting in sub-critical crack growth (SCG). The mode of crack propagation during SCG was essentially intergranular. Flexural stress-rupture evaluation in the temperature range 800 to 1000° C has identified the stress levels for time-dependent and time-independent failures.