The　failure　process　of　soldered　joint　in　SMT　was　investigated　by　electrical　resistance　measurement　method　and　crack　observation　method.　The　characteristics　of　electrical　resistance　value　variation　of　lead-tin　and　lead-free　soldered　(SAC305)　joints　during　the　thermal　fatigue　test　were　obtained.　And　at　the　same　time　the　crack　propagation　in　soldered　joint　was　observed.　According　to　these　measurements,　the　failure　rules　of　lead-tin　and　lead-free　soldered　joint　were　compared.　The　relationship　between　electrical　resistance　value　variation　and　crack　propagation　of　soldered　joint　during　thermal　fatigue　test　was　studied　by　FEM,　and　an　empirical　criterion　to　estimate　the　failure　of　the　soldered　joint　in　the　thermal　fatigue　test　was　obtained　based　on　electrical　resistance　value　variation.　The　experimental　results　show　that　the　lead-free　soldered　joint　has　a　higher　resistibility　in　thermal　fatigue　than　the　traditional　lead-tin　soldered　joint.　The　criterion　based　on　electrical　resistance　value　variation　was　founded　from　the　experimental　and　simulation　results.