Abstract: To decide the defect of composition segregation is difficult in the ultrasonic inspection of small diameters titanium alloy bars. Wandering echo is observed between the bottom wave and equilateral triangle wave on inspecting the TC4 diameter 9mm titanium alloy bars. Through dissecting the bars, there is composition segregation with alloying agent-depleted zone in the titanium alloy. The character of the defect is its acoustic impedance similar to that of parent mateial, which belongs to penetrant defect. The sound beam is attenuated while passing through the defect, resulting in that the echo wave appears between the primary bottom wave and equilateral triangle wave. There are two types of defects of composition segregation: one is penetrant defect whose acoustic impedance is similar to that of parent mateial,the other is reflected defect whose acoustic impedance is different to ones.It is important to observe the echo wave appearing between the primary bottom wave and equilateral triangle wave in oder to avoid failing to detect the penetrant defect of composition segregation.