A new exsolved copper-iron-sulfur phase in sphalerite has been found in the Dajing cassiterite- sulfidecopper-silver-tin deposit, Inner Mongolia, China. After investigating the optical properties, chemical composi-tion, crystal structure and exsolved Characters of the exsolved phase, the authors present important evidencefor its being a new exsolved copper-iron-sulfur phase mineral. Its formation mechenism is discussed in consider-ing heating experiments, phase equilibrium data and the geology of the deposit.