The expanding curves of BT25 titanium alloy under different cooling rates was investigated using dilatometry method. It was proved that the expansion method was able to measure accurately phase transformation of BT25 titanium alloy at different cooling rates. The continuous cooling transformation ( CCT ) diagram of this alloy was obtained by means of X ray diffractometry ( XRD), scanning electron microscopy(SEM) and micro-hardness。As well as the effect of cooling rate on microstructure evolution was investigated .The results showed that the Widmanstaten structure is obtained at the cooling rate of less than C1 (1~3 ℃?s -1); therefore, α" phase is generated at the cooling rate of greater than C2 (50 ~100 ℃?s -1) ; The microstructure consists of Widmanstaten structure and α" at the cooling rate from C1 to C2. With the cooling rate decreases, the quantity of Widmanstaten structure increases and the quantity of α" phase decreases in this microstructure. The micro-hardness of BT25 alloy decreases as the cooling rates decreasing.