The atomic configuration and electronic structure of erbium-doped silicon are studied theoretically by using local density formalism (LDF) and discrete variational method (DVM).The influence of codoping with oxygen is also investigated.The minimum-energy atomic configuration is that erbium occupies a tetrahedral interstitial site for an isolated erbium atom.In this configuration,Er 5d levels are located near the conduction band edge of Si in shallow resonance states. With the presence of oxygen,the binding energy of the structure decreases. In this case,the minimum-energy atomic configuration is that erbium occupies a hexagonal interstitial site with six nearest neighbors of oxygen atoms. An electronic state caused by the hybridization of Er 5d,O 2p and Si 2p electrons is located 0.3eV below the conduction band edge,which might be correlated with the deep level at Ec-0.15eV observed by the experimental measurement in Er and O codoped Si.