The effects of addition of Mn on age hardening behaviors of Cu-6wt.%Ni-6wt.%Sn were investigated. The microstructural and hardness changes of alloys aged at 350 to 550℃ were studied by optical microscopy and X-ray diffraction techniques. The Mn free alloy decomposed spinodally by aging at 350℃ followed by the formation of metastable γ` phase which accompanies the increase of hardness. Further aging transformed metastable γ` phase to cellular structure of α+γ. When the specimens were aged at 450℃ and above, cellular structure of α+γ formed without spinodal decomposition and transition of metastable phase. The additions of Mn affected the sequences of precipitation. Additions of Mn suppressed spinodal decomposition of the alloy in the early stage of aging and exhibited a incubation period in hardness change. Hardening was accompanied by the formation of MnNi₂Sn phase.