We determined the atomic configuration of Er-O complexes in silicon from the first principles using the super-cell FLAPW method within the local density approximation. We proposed two models of the atomic configuration of Er-O6 complexes in silicon with C 2 symmetry, one is stable and the other is meta-stable in the thermal equilibrium conditions. Our proposed atomic configurations of Er-O6 complexes are consistent with the experimental results of EPR and EXAFS measurements, which reported the existence of several stable and meta-stable low-symmetry centers and similarity to the C 2 -symmetry center of the Er 2 O 3 .