Gallium arsenide possesses excellent photoelectric property which have been widely used in electron and semiconductor industries. To promote the cleavage manufacturing technology of gallium arsenide, the anisotropy of mechanical property of gallium arsenide is calculated in this work. The structural parameters, such as angle between the crystal planes, the spacing between planes and density of atoms, were calculated. The change rule of elastic modulus, Poisson’s ratio, shear modulus, hardness and fracture toughness of monocrystalline gallium arsenide on crystal {100} plane with crystal orientation was analyzed by generalized Hooke’s law and indentation tests. The results show that difference of structural parameters between planes is the main reason for the anisotropy of mechanical properties of GaAs. With variation of crystal orientation, the elastic modulus and Poisson’s ratio on the {100} crystal plane appear periodic changes, while shear modulus is identically equal to 59.4 GPa. Anisotropy of GaAs {100} crystal plane hardness changes slightly while fracture toughness changes greatly with a minimum value of 0.304 MPa·m^1/2 in <110> crystal direction which is sensitive to the crack propagation.