Relation between magnetic anisotropy and MnSb layer thickness has been studied for MBE-grown MnSb/GaAs (001) system, in connection with the distortion of the MnSb hexagonal structure. For thick layers (d > 40 nm), c/a values are comparable to or slightly smaller than that of the free-standing bulk MnSb, and the MnSb [0001] is the primary magnetization easy axis. For thin layers (d < 40 nm), c/a values become larger than that of the bulk MnSb, and the MnSb [11<(2)over bar>0] (perpendicular to c-axis) becomes a new easy axis. At d=40 nm, both MnSb [0001] and MnSb [11 (2) over bar0] axes behave as the easy axis. These experimental results are explained consistently in terms of the dipole-dipole interaction. (c) 2012 Elsevier B.V. All rights reserved.