The effect of silicon nitride (Si3N4) addition on the sintering of magnesium silicon nitride (MgSiN2) powder has been examined using a hot-pressing technique. The MgSiN2 powder compacts with 1, 4, and 9 mol% of Si3N4 addition [sintering aid: 1 mol% ytterbium oxide (Yb2O3)] were hot-pressed at a temperature between 1550 and 1800 °C for 90 min in a nitrogen (N2) atmosphere under the pressure of 75 MPa. The relative density of a MgSiN2 compact with 4 mol% of Si3N4 addition hot-pressed at 1600 °C for 90 min showed a maximum (98.6%), although the relative density was reduced by the addition of Si3N4 at other hot-pressing temperatures. The maximum fracture toughness (6.6 MPa·m1/2) was achieved for the MgSiN2 specimen with 4 and 9 mol% of Si3N4 addition hot-pressed at 1600 °C for 90 min. This value was 2.5 times higher than that of MgSiN2 specimen without Si3N4 addition (2.6 MPa·m1/2). The improvement of the fracture toughness appeared to be due to the elongation of the Si3N4 grains, which had occurred in the presence of a liquid phase during the hot pressing. The thermal conductivity of the MgSiN2 specimen with 9 mol% of Si3N4 addition hot-pressed at 1750 °C for 90 min increased to 34.1 W m-1 K-1, which is to our knowledge, the highest among the values so far reported for MgSiN2 ceramics.