We have investigated the phase transition and structural properties of AlAs in three crystallographic structures, Le., B3 (zinc blende), B1 (rocksalt), and B8 (nickel arsenide), at high pressures using the full-potential linearized muffin-tin orbital (FP-LMTO) scheme within the generalized gradient approximation correction (GGA) in the frame of density functional theory (I)FT). For B8 structure, it is found that the c/a ratios kept nearly constant (similar to 0.2% fluctuation) corresponding to V/V-0 similar to 0.7-1.05 (V is the primitive cell volume and V-0 is the experimental equilibrium volume of B3 structure), which is in full agreement with experiment, but the c/a ratios increase linearly with the values of V/V-0 decreasing corresponding to V/V-0 similar to 0.4-0.7. This indicates under low pressure the compression along c-axis and a-axis is the same, but the compression along c-axis is more difficult than along a-axis under higher pressure. Based on the condition of equal enthalpies AlAs is found to undergo a structural phase transition from B3 to B8 at 5.34 GPa, in agreement with the experimental value of 7 +/- 5 GPa, and is speculated to undergo the B3-B1 transition at 6.24 GPa. (C) 2009 Elsevier B.V. All rights reserved.