Densification and thermal stability of hot-pressed Si3N4-ZrB2 ceramics with and without additives were investigated in N-2 atmosphere. The addition of MgO-Yb2O3, MgO-Y2O3, and Al2O3-Yb2O3 resulted in significant increase in relative density of the ceramics hot-pressed at 1500 degrees C from 48.5% to 98.0%, 97.3%, and 95.6%, respectively. There was weak reaction of ZrB2 with N-2 to form ZrN in hot-pressed ceramics. Then heat treatment at 1550 degrees C resulted in the further reactions to produce ZrN, ZrSi2, and BN. The Si3N4-ZrB2 ceramics with MgO-Yb2O3 showed much better thermal stability as compared to the ceramics with Al2O3-Yb2O3. The small difference in density led to the obvious difference in thermal stability. Therefore, Si3N4-ZrB2 ceramics should be densified to full density, to obtain high thermal stability.