A specimen of Cu-35%Sn alloy has been subjected to the unidirectional heat treatment in an attempt to examine the evolution of microstructures under varying thermal conditions. The specimen was cast in the form of a cylinder 10 ㎜ in diameter and 200 ㎜ in length, which was then installed in the temperature gradient field established inside a vertical tube furnace. The furnace temperature was adjusted to make the upper part at 750℃ and bottom end part at 300℃ of the specimen. The experiment was terminated by dropping it into water after the 30 minutes holding at given temperature. By the rapid cooling, the high temperature phases, γ and ζ, were retained at ambient temperature with some of γ phase transformed to E phase, especially at the grain boundaries of γ phase. The presence of E phase was found to determine the nature of phase transformations of the ζ phase undergoes upon cooling. In the close area of the E phase, E phase grew separately out of ζ and adds to the preexisting E whereas in areas away from ε₁, both δ and E grew simultaneously out of ζ and formed a lamella eutectoid structure. The transformation to δ was found to occur only in slow cooling. The hardness on each phase showed that the retained phases, γ and ζ, could be plastically deformed without brittle fracture while the phases, E and 6, were too brittle to be deformed.