Because of its large surface area, nanosized zinc sulfide (Nano-ZnS) has been demonstrated in a previous study to be efficient for removal of elemental mercury (Hg0) from coal combustion flue gas. The excellent mercury adsorption performance of Nano-ZnS was found to be insusceptible to water vapor, sulfur dioxide, and hydrogen chloride. However, nitrogen oxides (NOX) apparently inhibited mercury removal by Nano-ZnS; this finding was unlike those of many studies on the promotional effect of NOX on Hg0 removal by other sorbents. The negative effect of NOX on Hg0 adsorption over Nano-ZnS was systematically investigated in this study. Two mechanisms were identified as primarily responsible for the inhibitive effect of NOX on Hg0 adsorption over Nano-ZnS: (1) active sulfur sites on Nano-ZnS were oxidized to inactive sulfate by NOX; and (2) the chemisorbed mercury, i.e., HgS, was reduced to Hg0 by NOX. This new insight into the role of NOX in Hg0 adsorption over Nano-ZnS can help to optimize operating conditions, maximize Hg0 adsorption, and facilitate the application of Nano-ZnS as a superior alternative to activated carbon for Hg0 removal using existing particulate matter control devices in power plants.