Abstract：It was considered before that Cd can only limitedly substitute Zn in sphalerite in nature. However, in the recent study of the relationships between Cd and Zn in the Nujiaotang cadmium-zinc deposit, Guizhou, using the electron microprobe, scanning electron microscope and transmission electron microscope, the authors have found that Cd in zinc sulfide mainly exists as an isomorphous element with its content ranging from 0.8% to 37.93%, and that likewise Zn in cadmium sulfide also mainly exists as an isomorphous element with its content ranging from 2.43% to 38.64%. Zinc sulfide and cadmium sulfide show gradational relationship, and there is a good negative correlation between Zn and Cd with a correlation coefficient of > 0.99, forming a ZnS-CdS complete isomorphous series. This discovery has great theoretical significance and practical value not only in mineralogy but also in geochemistry and geology of mineral deposits.