Dynamic and static fatigue behavior of a hotisostatically pressed (HIPed) silicon nitride was investigated at 1150, 1260 and 1370°C. Uniaxial tensile tests were conducted over ranges of constant stresses and constant stress rates. Correlation of stress-life relations between static and dynamic fatigue results was evaluated and failure modes were determined as functions of temperature, stress and stress rate. At 1150°C the static and dynamic fatigue failures were controlled by a slow crack growth mechanism for all stresses and stress rates. Creep rupture was the dominant failure mechanism in static loading at 1260 and 1370°C. A transition in the dominant failure mechanism in dynamic fatigue at 1260 and 1370°C occurred at a stress rate of 10-2 M Pa/s. Slow crack growth was the dominant failure mechanism with stress rates > 10-2 M Pa/s while creep rupture was the governing mechanism for the failure at stress rates ≤10-2 M Pa/s.