The phase transformation behavior of Ti-42Al-5Mn (at.%) alloy from different phase regions with various cooling rates was investigated based on electron probe micro analyzer-backscattered electrons (EPMA-BSE). It is shown that β→α₂′ takes place when this alloy is cooled at a high rate, such as water quenching (WQ), oil cooling (OC), from β single phase. With the decreasing cooling rate to air cooling (AC), β→α₂′ is restrained and β→γ is promoted by forming γ platelets. The room-temperature microstructure is β_o + α₂ when alloy cooled (WQ and OC) from (β + α) dual-phase. However, under AC, β→γ occurs and γ platelets form. It should be noted that α₂→γ happens when this alloy cooled from 1180 °C (>T_eut) by OC and AC, forming an incomplete lamellae (α₂/γ) structure in the α₂ phase. However, when the alloy cooled from 1100 °C (<T_eut), α₂/γ→β_o,sec occurs and complete lamellae generates in α₂ phase.