This paper reports the gas pressure sintering and thermal conductivity of silicon nitride with 2 mol% Y₂O₃ and varied contents of MgSiN₂ in the range of 0 to 5 mol% at 1900°C under a nitrogen pressure of 1 MPa N₂. It was found that the density curves show a typical "V" shape against the MgSiN₂ content. The 1 mol% MgSiN₂ hinders significantly the densification, but the 5 mol% MgSiN₂ leads to complete densification. The inhibited densification is associated with the pronounced normal grain growth. However, the abnormal grain growth is inhibited with the addition of MgSiN₂. After sintering for 12 h, the sample without MgSiN₂ exhibits the coarsest and highest bimodal microstructure, whereas the one with 5 mol% MgSiN₂ exhibits the finest microstructure. Although the MgSiN₂ addition leads to a slightly enhanced thermal conductivity, it tends to reduce the thermal diffusivity. The thermal conductivity without porosity shows a decrease with the increase of MgSiN₂ content because of the inhibited grain growth.