Ultrasonic　C-scan　of　spot　weld　on　galvanized　sheet　is　carried　out　by　using　ultrasonic　water　immersion　focusing　method.　The　A-scan　signals　of　the　specific　parts　of　the　ultrasonic　C-scan　image　are　analyzed.　C-scan　can　be　adopted　to　measure　the　diameter　of　the　spot　weld　and　discover　the　inner　flaws　of　the　spot　weld　such　as　splash,　crack,　void　and　inclusion.　Besides,　C-scan　can　detect　the　appearance　of　the　spot　weld　too.　In　order　to　acquire　the　more　accurate　information　of　the　inner　structure　of　the　spot　weld,　the　characteristic　C-scan　image　and　the　characteristic　A-scan　signals　which　can　be　used　to　distinguish　the　shape　and　type　of　inner　defects　are　also　obtained.　The　ultrasonic　C-scan　image　method　not　only　can　evaluate　the　quality　of　spot　weld　but　also　can　be　a　kind　of　important　auxiliary　method　for　spot　weld　research.　©　2009　Journal　of　Mechanical　Engineering.