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Polymer Bonded Explosives (PBX) are well-known explosives for the weapons, especially for the parts of some important weapon. Generally, they first are modelled in cylinder. One of the most important issues in PBX cylinder properties is the detection of its flaw. Each PBX cylinder may have flaws that influence the security, machine-performance and other important physical or chemical performance. Its detection can be made by ultrasonic testing techniques. The ultrasonic (US) testing is based on the fact that the ultrasonic wave is able to distinguish different acoustic impedances when there is the different medium inside the cylinder. The special purpose sensors ranging from 1.25MHz to 5MHz were used to emit/receive the ultrasonic signals. When the US pulse wave propagates from the first surface in the PBX, part of the pulse is propagated back to the transducer owed to the flaw interface. The other part of US pulse wave goes forward and reaches the next surface, this signal eventually goes back to the transducer. These signals containing the flaw information are recorded for analysis. Based on processing the signals by means of the wavelet transform, we can distinguish the flaw with the frequency change of flaw echo and verify the central frequency of the transducer, obtain the accurate part frequency of the signals with which is concerned, and also accurately measure the signal absolute value (amplitude) depending on the effective frequency. The accuracy obtained will encourage its implementation for nondestructive testing of the PBX's flaw.