Mineral sulfide based sorbents were superior alternatives to traditional activated carbons for elemental mercury (Hg0) immobilization in industrial flue gas. A systematical study concerning the influence of sulfur trioxide (SO3) on Hg0 adsorption over a nanosized copper sulfide (Nano-CuS) was for the first time conducted. SO3 was found to significantly inhibit the Hg0 removal over Nano-CuS partially because SO3 oxidized the reduced sulfur species (sulfide) with high affinity to mercury to its oxidized sulfur species (sulfate). Moreover, a brand new "oxidation-reduction" mechanism that led to a simultaneous oxidation of sulfide and reduction of mercury on the immobilized mercury sulfide (HgS) was responsible for the inhibitory effect. Even though the released Hg0 from the reduction of mercury in HgS could be oxidized by SO3 into its sulfate form (HgSO4) and recaptured by the sorbent, the "oxidation-reduction" mechanism still compromised the Hg0 capture performance of the Nano-CuS because HgSO4 deposited on the sorbent surface could be easily leached out when environmentally exposed. These new insights into the role of SO3 in Hg0 capture over Nano-CuS can help to determine possible solutions and facilitate the application of mineral sulfide sorbents as outstanding alternatives to activated carbons for Hg0 immobilization in industrial flue gas.