Phase separation behaviors of a quenched Cu-3.0at%Ti alloy, as well as crystallographic structures of Cu-20.7at%Ti alloy have been studied using transmission electron microscopy. The furnace-cooled Cu-20.7at%Ti alloy are composed of alpha-Cu(4)Ti (Ni(4)Mo-type) and beta-Cu(4)Ti (Au(4)Zr-type) with the orientation relationship of (011)(alpha)//(110)(beta), [100](alpha)//[001](beta). As-quenched Cu-3.Oat%Ti alloy showed a modulated structure with the modulation length of about 4 nm. When aged at 723K for 8 hr, the alpha-Cu(4)Ti phase emerges within the modulated or tweed-like microstructure. Prolonged aging results in the growth of the alpha-Cu(4)Ti particles and the loss of coherency. It is likely that as-quenched Cu3.0at%Ti alloy decomposes spinodally at 723K, followed by polymorphous ordering; though the present study did not exclude, as an alternative path, a decomposition mechanism based on the catastrophic nucleation.