Abstract: Laser powder bed fusion (LPBF) is used to fabricate complex-shaped, dense, and high-performance oxide ceramics. During LPBF, bubbles form and evolve in the melt pool and ultimately remain in the printed ceramics as pores, which significantly degrade the mechanical properties. Therefore, it is essential to understand the bubble behaviors during LPBF. Herein, we conducted an in-situ investigation of the bubble dynamics in the melt pool of homogeneously mixed Al₂O₃–Y₂O₃ powders using synchrotron high-speed X-ray imaging. The formation, growth, motion, and evolution of bubbles, as well as the relationship between the instability of melt flow and bubble rupture during LPBF, were elucidated. The findings reveal that bubbles from the interstices within the powder bed grow following three distinct modes, i.e., uplift growth, gas channel attachment, and bubble coalescence. Furthermore, melt flow oscillations caused by the bursting of large bubbles can lead to local instability of the melt pool. Results from this study enhance the understanding of bubble dynamics during LPBF and may provide valuable insights for pore elimination in LPBF-processed oxide ceramics.