The　successful　detection　of　defects　in　a　structure　using　non-destructive　testing　(NDT)　is　very　significant　to　maintain　a　reliable　operation　of　the　structure.　Thus　the　reliability　of　the　NDT　method　becomes　a　critical　factor　that　influences　the　reliability　evaluation　of　the　structure.　A　brief　review　on　the　NDT　reliability　literature　is　presented　in　the　first　section　of　the　paper.　The　reliability　operating　characteristic　curve　of　ultrasonic　contact　testing　and　immersion　testing　of　frictional　bonded　piping　joints　is　plotted　using　the　signal-to-noise　ratio　(SNR)　as　a　criterion.　It　shows　that　ultrasonic　testing　of　these　specimens　has　a　higher　reliability　in　the　detection　of　macro-defects　than　micro-defects.　Immersion　testing　has　a　higher　reliability　than　contact　testing.　Digital　signal　processing　(DSP)　technology　has　the　potential　to　increase　the　SNR　of　the　ultrasonic　signal.　A　traditional　DSP　routine,　band-pass　filtering,　and　the　most　recent　DSP　method,　wavelet　transform,　are　utilised　to　enhance　the　SNR　of　signals,　to　increase　the　reliability　of　the　ultrasonic　testing.　The　results　indicate　that　after　the　SNR　of　signals　is　enhanced　by　a　suitable　digital　signal　processing　technique,　the　reliability　of　the　ultrasonic　testing　increases.