This paper deals with the alpha- to beta- Si3N4 phase transformation during pressureless sintering of Si3N4 with LiYO2 as a sintering additive. The phase evolution after heat treatments in the temperature range from 1200 to 1500degreesC shows that alpha-Si3N4 and beta-Si3N4 are the main crystalline phases in all stages of annealing. At 1200degreesC, the initial LiYO2 sintering additive completely disappears. X-ray patterns show a remarkable broadening of diffraction lines in the regions around 27-29degrees and 32-34degrees 2theta, which can be ascribed to the Y-5(SiO4)(3)N ("N-apatite") phase. This indicates that the nucleation and formation of crystalline N-apatite takes place at this temperature, which is the onset of liquid formation. At 1600degreesC, a complete conversion of the secondary phase into the liquid one and of alpha- to beta-Si3N4 takes place.