In　the　present　paper　ultrasonic　testing　of　spot　welds　is　briefly　reviewed　for　evaluating　the　size　or　quality　of　spot　welds.　In　this　work,　a　zinc-coated　high　strength　steel　sheet　DP　600　is　used　for　the　spot　welding.　A　wavelet　packet　transform　(WPT)　processing　is　adopted　to　analyze　the　ultrasonic　signals　acquired　by　the　pulse　echo　from　spot　weld.　For　the　purpose　of　nondestructive　evaluation　to　the　spot　weld,　the　work　is　to　establish　the　relation　between　ultrasonic　signals　and　nugget　diameter　or　relevant　maximum　shear　load.　The　results　indicate　that　the　characteristic　value　in　relation　with　the　nugget　diameter　and　maximum　shear　load　of　spot　weld　is　found　on　a　basis　of　decomposed　signals　in　different　scale　in　WPT　Moreover,　the　characteristic　value　is　indicated　by　the　power　of　signals　decomposed　in　WPT.　The　nugget　diameter　and　maximum　shear　load　are　decreased　with　increasing　the　characteristic　value.　Moreover,　it　is　found　that　the　tensile　stress　of　the　spot　welds　has　no　clear　relation　with　the　value　of　characteristic　signal.　Furthermore,　the　obtained　characteristic　value　may　be　an　important　basis　for　an　automatic　recognizing　of　weld　size　and　maximum　shear　load.　In　addition,　spatial　averaging　was　employed　to　decline　the　stochastic　disturbance　and　effect　of　the　human　factor　on　the　signal　before　WPT.　(c)　2008　Elsevier　B.V.　All　rights　reserved.