Generally, flotation is used for the mineral processing of copper sulfide minerals such as chalcocite, bornite, and chalcopyrite. These copper sulfide minerals are high floatability minerals, thus they can be recovered as a froth product just by adding a collector. When the surfaces of copper sulfide minerals are oxidized, however their floatability decline and it is difficult to recover and concentrate them by only collector flotation. In this case, flotation reagents that function as activator are added to improve their floatability. Sodium hydrosulfide (NaHS) is known as activator to have the effects of re-sulfurizing and activating oxidized surfaces of chalcocite and bornite. In this study, the effect of pH and NaHS dosage on the flotation of a complex copper ore was examined. Flotation tests were conducted on a Chilean copper ore containing chalcocite, bornite, and chalcopyrite using a Denver-type flotation cell at 0, 50, 100 g/t NaHS and pH 8, 9, 10. At several points in the flotation tests, solutions were sampled for analysis of S ion in the solutions. From the solution analyses and copper flotation results, the reaction of NaHS, as well as the influence of pH and NaHS dosage on copper recovery was discussed.