The Pr alpha-sialon powders prepared by self-propagating high-temperature synthesis (SHS), consisting of 55 wt% Pr alpha-sialon and 45 wt% of beta-sialon (abbreviated as alpha' and beta'), were hot-pressed at 1800 degrees C for 1 h. The results showed that Pr alpha' phase would transfer to beta' with the appearance of JEM phase (Pr(Si6-zAlz)(N10-zOz)) after sintering, thus resulting in the increase of beta' phase to 86 wt%. The addition of Y2O3 into SHS-ed Pr alpha' powders as the starting materials restrains the transformation of alpha' to beta' and prevents the formation of JEM phase as well. The nucleation mechanism of Pr alpha' grain during hot-pressing was investigated in terms of transmission electron microscope and energy-dispersive spectrometer analysis. Two nucleation modes of Pr alpha' grains were found, i.e., nucleating on the undissolved Pr alpha' grains and on the nuclei of (Pr, Y) alpha' grains precipitated from liquid phase.