A study has been made on the roasting reactions of copper bearing nickel mattes. Typical three kinds of artificial mattes were prepared, and various changes occurring in roasting of them were measured. There is a very noticeable difference between the results on matte and that obtained on mixture having the same composition. It is due to the fact that the primary crystallized Cu2S in matte is liberated from Ni3S9 by grinding and oxidized as same as Cu2S itself but the eutectic Cu2S is oxidized with Ni3S2 crystal.