The influence of Cu addition on MnS precipitation and growth behavior in Fe-2.88Si-0.1Mn-xCu (x = 0, 0.25) has been investigated. SEM observation clarified that Cu addition has no effect on the morphology of MnS or its growth rate at early stage. In contrast, Cu addition suppresses the growth rate at the Ostwald ripening stage. At the early stage of growth, a plateau region was observed both with and without Cu addition. Neither the conventional diffusion limited growth nor the Ostwald ripening theory adequately explains these observations. Using the Kampmann-Wagner Numerical (KWN) model, it was possible to successfully reproduce such behavior, including the plateau region. The interfacial energy between ferrite and the MnS was evaluated by the KWN-model, and good agreement was found with previously reported values. The suppression in the growth rate at the Ostwald ripening stage by Cu addition can be explained by a 15 pct reduction in the interfacial energy which possibly occurs due to the segregation of Cu at the interface between MnS and ferrite. This segregation was experimentally confirmed by STEM-EDX analysis.