We use neutron polarization analysis to study temperature dependence of the spin excitation anisotropy in BaFe 2 As 2 , which has a tetragonal-to-orthorhombic structural distortion at T s and antiferromagnetic (AF) phase transition at T N with ordered moments along the orthorhombic a axis below T s ≈ T N ≈ 136 K. In the paramagnetic tetragonal state at 160 K, spin excitations are isotropic in spin space with M a

M b

M c , where M a ,

M b , and M c are spin excitations polarized along the a -, b -, and c -axis directions of the orthorhombic lattice, respectively. On cooling towards T N , significant spin excitation anisotropy with M a

M b ≈ M c develops below 3 meV with a diverging M a at T N . The in-plane spin excitation anisotropy in the tetragonal phase of BaFe 2 As 2 is similar to those seen in the tetragonal phase of its electron and hole-doped superconductors, suggesting that spin excitation anisotropy is a direct probe of doping dependence of spin-orbit coupling and its connection to superconductivity in iron pnictides.