In order to investigate the mechanism of dissolutin of nickel from a matte composed essentially of heazlewoodite and ferronickel, the leaching in an aqueous solution of hydrochloric was carried out at 50-90℃, 0.5-3.ON HCl, and with the particle sizes between 100-160 μm and 400-500 μm. The leaching rate of heazlewoodite was much rapider than that of ferronickel, and the two components were simultaneously dissolved with different reaction rates. Within the experimental range, NiS, an intermediate, was not formed. However a possibility to form a thin layer of NiS on particle surface upon saturating the reaction mixture with H₂S was not excluded. At the initial stage, the activation energy·was about 10 kcal/g mol. The reaction occurred at the particle surface, and consequently the particle size decreased with time. The rate of dissoultion for nickel was linearly proportional to the concentration of HCl up to1.0N.