Four different Si3N4 ceramics using 10 wt% celsian (BaAl2Si2O8) as an additive, have been prepared by hot pressing. Different celsian sources and hot-pressing conditions were used and their effects on densification, in situ crystallization of celsian, alpha to beta phase transformation, microstructures and properties, were examined. The use of a presynthesized celsian as a raw material was found to enhance the rates of densification and phase transformation due to the excess silica from raw Si3N4 powders which, however, did not change the microstructure and crystallization behaviour of celsian. Increases in hot-pressing temperatures and times increased the total number of large elongated beta grains and resulted in coincidental increases in strength and toughness until the alpha to beta transformation was complete. An intermediate quench-and-reheat step during hot pressing made the microstructure finer and more uniform by greatly reducing the large-sized elongated beta grains. The quenching step also disrupted complete crystallization of celsian which led to more grain-boundary glassy phase and compromised the material’s high-temperature properties.