Quantum chemical calculation is an important method to investigate the molecular structures for multi-atom molecules. The determination of electronic configurations and the accurate description of the symmetry of molecular orbitals are critical for understanding molecular structures. For the molecules belonging to high symmetry group, in the quantum chemical calculation the sub-group is always adopted. Thus the symmetries of some electric states or some molecular orbitals, which belong to different types of representations of high symmetry group, may coincide in the sub-group presentations. Therefore, they cannot be distinguished directly from the sub-group results. In this paper, we provide a method to identify the symmetry of molecular orbitals from the theoretical sub-group results and use this method to determine the symmetry of the highest occupied molecular orbitals (HOMO) of the sulfur hexafluoride SF₆ molecule as an example. Especially, as a good insulating material, an important greenhouse gas and a hyper-valent molecule with the high octahedral O_h symmetry, SF₆ has received wide attention for both the fundamental scientific interest and practical industrial applications. Theoretical work shows that the electronic configuration of ground electronic state ^1\rm A_1g of SF₆ is (\rm core)^22(4\rm a_1\rm g)^2(3\rm t_1\rm u)^6(2\rm e_\rm g)^4(5\rm a_1\rm g)^2(4\rm t_1\rm u)^6(1\rm t_2\rm g)^6(3\rm e_\rm g)^4(1\rm t_2\rm u)^6(5\rm t_1\rm u)^6(1\rm t_1\rm g)^6 and the symmetry of the HOMOs is T_1g . However, in some literature, the symmetry of HOMOs of SF₆ has been written as T_2g instead of T_1g . The reason for this mistake lies in the fact that in the ab initial quantum chemical calculation used is the Abelian group D_2h , which is the sub-group of O_h , to describe the symmetries of molecular orbitals of SF₆. However, there does not exist the one-to-one matching relationship between the representations of D_2h group and those of O_h group. For example, both irreducible representations T_1g and T_2g of O_h group are reduced to the sum of B_1g , B_2g and B_3g of D_2h . So the symmetry of the orbitals needs to be investigated further to identify whether it is T_1g or T_2g . In this work, we calculate the orbital functions in the equilibrium structure of ground state of SF₆ by using HF/6-311G* method, which is implemented by using the Molpro software. The expressions of the HOMO functions which are triplet degenerate in energy are obtained. Then by exerting the symmetric operations of O_h group on three HOMO functions, we obtain their matrix representations and thus their characters. Finally, the symmetry of the HOMOs is verified to be T_1g . By using this process, we may determine the molecular orbital symmetry of any other molecules with high symmetry group.