Abstract: In order to investigate the liquefaction characteristics of saturated coral sand from Nansha Islands under different initial major stress direction angles (α₀), a series of undrained cyclic torsional shear tests for saturated coral sand are carried out by using the GDS hollow cylinder torsional apparatus. A remarkable finding of this study is that, α₀ has significant influence on the liquefaction characteristics of coral sand, the excess pore water pressure (u_e) for case α₀ ≠ 45° cannot reach the initial mean principal stress ${p^{\prime }}_0$, but the u_e for case α₀ = 45° can eventually reach do. The developments of strain components for various α₀ cases are different, but the specimens for all α₀ cases will produce larger general shear strain amplitude (γ_ga), and the correlation between the u_e to ${p^{\prime }}_0$ and γ_ga can be expressed approximately as tangent function. Based on the experimental data, the number of cycles (N_L) required to induce initial liquefaction decreases with the increasing α₀ for a given cyclic stress ratio (CSR). However, by defining the modified cyclic stress ratio (CSR_M) as a new index for cyclic stress level, a virtually unique relationship between CSR_M and N_L can be established for all α₀ cases. The applicability of CSR_M is verified by the original test data of saturated sands from the literatures.