According to EET theory and BLD method,the valence electron structures of RE compound with O and S,such as,RE2O3,REO2,RE2O2S,RE2S3,RES,were calculated.The property of RE compound with O and S were explained by the correspondent valence electron structure.The forming consequence and condition of RE compound in steel were proved.The results indicate that low temperature is helpful for RE to deep desulphurization.