The effect of L-cysteine in different concentrations on the bioleaching of Ni-Cu sulfide and marmatite were studied with a moderately thermophilic, sulfur-oxidizing bacterium, strain of Acidithiobacillus caldus. X-ray diffraction (XRD) observations showed the change of bioleached solid residues and the effect of L-cysteine on the surface charges of minerals. It was found that adding certain amounts of L-cysteine to the leaching system of Ni-Cu sulfide largely enhanced the leaching rate, while L-cysteine inhibited the bioleaching of marmatite by A. caldus. The mechanism of L-cysteine interaction with mineral surfaces was studied by means of zeta potential determination and IR spectra.