Constant stress compressive creep studies have been conducted on hot-pressed Si3N4 containing 30 vol. % SiC whiskers and an initial vitreous phase composed of Al2O3, Y2O3, and SiO2. The conditions of temperature and stress were 1470–1670 K and 50–350 MPa, respectively; the atmosphere was purified N2 at 1 atm. Significant changes in the stress exponent and activation energy indicate a change in the controlling creep mechanism at ≍225 MPa and 1570 K. Prolonged annealing in the unstressed condition reduced creep rates but had little effect on the stress exponent values. Transmission and scanning electron microscopy revealed that the break in the stress exponent curves was caused by the removal of the amorphous material at the grain boundary and resulting contacts between Si3N4 grains. The break in the activation energy curves is believed to be similarly related. Analysis of all the data indicates that the composite creeps via grain boundary sliding accommodated by viscous flow at low stresses and temperatures and by diffusion at high stresses and temperatures. The contributions of these two mechanisms varied measurably as a function of stress and temperature. No cavitation was observed. The presence of the SiC whiskers had no observable effect on deformation.