The pressure-induced structural evolution of apatite-type La9.33Si6O26 was systematically studied using in situ syn-chrotron x-ray diffraction (XRD). The XRD spectra indicated that a subtly reversible phase transition from P63/m to P63 symmetry occurred at ～13.6 GPa because of the tilting of the SiO4 tetrahedra under compression. Furthermore, the La9.33Si6O26 exhibited a higher axial compression ratio for the a-axis than the c-axis, owing to the different axial arrange-ment of the SiO4 tetrahedra. Interestingly, the high-pressure phase showed compressibility unusually higher than that of the initial phase, suggesting that the low P63 symmetry provided more degrees of freedom. Moreover, the La9.33Si6O26 exhibited a lower phase transition pressure (PT) and a higher lattice compression than La10Si6O27. Comparisons between La9.33Si6O26 and La10Si6O27 provided a deeper understanding of the effect of interstitial oxygen atoms on the structural evolution of apatite-type lanthanum silicates (ATLSs).