The role of microstructure in affecting the fatigue crack growth resistance of grain bridging silicon nitride ceramics doped with rare earth (RE=Y, La, Lu) oxide sintering additives was investigated. Three silicon nitride ceramics were prepared using MgO-RE2O3 and results were compared with a commercial Al2O3-Y2O3-doped material. Decreasing stress intensity range (?K) fatigue tests were conducted using compact-tension specimens to measure steady-state fatigue crack growth rates. Specimens doped with MgO-RE2O3 additives showed a significantly higher resistance to crack growth than those with Al2O3-Y2O3 additives and this difference was attributed to the much higher grain aspect ratio for the MgO-RE2O3-doped ceramics. When the crack growth data were normalized with respect to the total contribution of toughening by bridging determined from the monotonically loaded R-curves, the differences in fatigue resistance were greatly reduced with the data overlapping considerably. Finally, all of the MgO-RE2O3-doped silicon nitrides displayed similar steady-state fatigue crack growth behavior suggesting that they are relatively insensitive to the intergranular film.