Si3N4 materials with distinct microstructures were prepared by hot-pressing, varying the holding time at the maximum temperature, and using different types and amounts of sintering additives. Materials with thermal conductivities of 15-82 W.(m.K)(-1) were obtained by changing the processing variables. The highest conductivity was measured for the material with the coarsest microstructure. The effect of microstructural parameters, such as percentage of secondary phases, grain size, and texture on thermal properties of Si3N4 ceramics, were studied. Hot-pressed Si3N4 ceramics were modeled as a two-phase composite made of large grains of high conductivity, and a small-grained phase of low conductivity.