The complexity of gas pressure liquid phase sintering (GPS-LPS) of silicon nitride induces a certain savoir-faire. This highly challenging strong bond ceramic uses sintering additives of a liquid phase sintering under high temperatures close to the silicon nitride phase decomposition which requires a gas pressure. The sintering behavior encompasses various phenomena such as Ostwald ripening grain growth, phase transition, and swelling when the pore elimination approach close porosity. The aim of this study was to model such sintering behavior. In this aim, master sintering curves (MSC) were used to study the temperature and porosity dependence of the sintering model. Then, the grain growth sensitivity of the sintering model was incorporated. Finally, the swelling phenomenon responsible for transient de-densification and sintering rate limiting mechanism at final stage sintering was extracted by an inverse method. A comprehensive sintering model reproducing all the silicon nitride sintering complexity result this study.