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Compressive creep behavior of Si_3N_4-3.62 mol% Y_2O_3-6.04 mol% Al_2O_3 hot-pressed at 1900℃ for 0.15 and 6h was investigated in the temperature range of 1250-1350℃ under 30-100 MPa, respectively. The relationship between the steady state creep rate and the grain size was discussed in relation to the grain growth mechanism during sintering. The grain size increased and then the creep rate decreased with increasing sintering time. The stress exponent of the creep rate was determined to be nearly unity. The apparent activation energy was measured to be about 700 kJ/mol. In addition, the microstructural observation revealed that no cavity appeared at grain boundaries during creep test. These results confirmed that the creep deformation in the apparent steady state creep regime is due to the diffusion-controlled solution/precipitation. Moreover, the grain size exponent in the creep equation was determined to be 2.4-3.1 and also found to be approached to 3 when the average of the smallest grains was selected. These findings indicate that the creep deformation is controlled by the diffusion through the grain boundary glassy phase, and that the solution/precipitation is mainly due to the small grain.