The effect of grain size on the tensile creep of silicon nitride was investigated on two materials, one containing 5% by volume Yb2O3, the other containing 5% by volume Yb2O3 and 0.5% by mass Al2O3. Annealing of the Al2O3-free silicon nitride for a longer period during processing increased the grain size by a factor of 2. This increase did not affect the tensile creep rate; the grain size exponent of the creep rate differed little from zero, p = -0.20 +/- 1.37 (95% confidence level). This finding supports the more recent theories of tensile creep for which p = 0 or -1 and rejects the more classical theory of solution-precipitation. In compression, a more limited data set showed p = -1.89 +/- 1.72 (95 % confidence level). In contrast to the Al2O3-free material, a longer term anneal of the Al2O3-containing material during processing did not increase its grain size. Despite this, the longer-annealed Al2O3-containing material crept 10 to 100 times slower than the short-annealed material. The enhancement of creep resistance may be a consequence of SiAlON formation during the additional annealing, which reduces the Al content in the amorphous phase and increases its viscosity. Such changes in chemical composition of the grain boundaries are more effective in controlling tensile creep rate than is the grain size.