A nanosized beta-Si3N4-based powder doped with sintering additives of 6 wt% Al2O3 and 8 wt% Y2O3 were sintered by spark plasma sintering at different heating rates. The in situ shrinkage curves were measured to identify the densification mechanism during the rise in temperature. At slower heating rates (50 and 100 degrees C/min), the nanosized grains are maintained after sintering at 1600 degrees C for 5 min, while anisotropic grain growth is accelerated above 1500 degrees C by applying a rapid heating cycle (200 degrees C/min). In addition to the dynamic Ostwald ripening that occur during the sintering process, the presence of Morie fringes and dislocations is attributed to grain rotation and misfit strain between the subgrains and elongated, large grains. Grain coalescence is found to occur in the rapid heating process.