The CAS phase is a major constituent phase for the continental crust and basaltic compositions at the P-T conditions of the Earth’s mantle, and potentially plays an important role in the geodynamic processes related to slab subduction. Its equation of state has been investigated here at ambient temperature up to about 25 GPa by using a diamond-anvil cell and synchrotron X-ray radiation. Its P-V data, fitted to the third-order Birch-Murnaghan equation, yield an isothermal bulk modulus (K _T ) of 185 (9) GPa and first pressure derivative (K′ _T ) of 7.2 (12). If K′ _T is fixed at 4, the derived K _T is 212 (4) GPa. Additionally, the CAS phase is strongly elastically anisotropic, with its a-axis direction much less compressible than c-axis direction: K _T−a :K _T−c = 2.19.