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 mm in diameter and 200 mm 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 $750^{circ}C$ and bottom end part at $300^{circ}C$ 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, ${gamma}$ and ${zeta}$, were retained at ambient temperature with some of ${gamma}$ phase transformed to ${varepsilon}$ phase, especially at the grain boundaries of ${gamma}$ phase. The presence of ${varepsilon}$ phase was found to determine the nature of phase transformations of the ${zeta}$ phase undergoes upon cooling. In the close area of the ${varepsilon}$ phase, ${varepsilon}$ phase grew separately out of ${zeta}$, and adds to the preexisting ${varepsilon}$ whereas in areas away from ${varepsilon}$, both ${delta}$ and ${varepsilon}$ grew simultaneously out of ${zeta}$, and formed a lamella eutectoid structure. The transformation to ${delta}$ was found to occur only in slow cooling. The hardness on each phase showed that the retained phases, ${gamma}$ and ${zeta}$, could be plastically deformed without brittle fracture while the phases, ${varepsilon}$ and ${delta}$, were too brittle to be deformed.