Sheet metal undergoes different strain conditions during different forming processes.The investigation of mechanical properties under these conditions is very important in the forming techniques.Sheet metal is particularly liable to failure under plain strain state.Measure and investigate the necking strain under plane strain condition is a particularly important study for sheet formability forecasting.In this study,material behavior of DP780 high strength steel sheet under plain strain condition was studied.Conventional plane strain tensile tests were carried out on the MTS testing machine with a special designed specimen.A digital image correlation system was employed to measure the full-field strain distribution during plain strain tensile test.The strain evolution during deformation was obtained and investigated.The capability of the specimen for plane strain test was validated from the strain distributions.The necking strain and fracture strain of DP780 high strength steel sheet were determined from the strain field and strain history results. Sheet metal undergoes different strain conditions during different forming processes. The investigation of mechanical properties under these conditions is very important in the forming techniques. Sheet metal is particularly liable to failure under plain strain state. a particularly important study for sheet formability forecasting. In this study, material behavior of DP780 high strength steel sheet under plain strain condition was studied. Conventional plane strain tensile tests were carried out on the MTS testing machine with a special designed specimen. A digital image correlation system was employed to measure the full-field strain distribution during plain strain tensile test.The strain evolution during deformation was obtained and investigated. the capability of the specimen for plane strain test was validated from the strain distributions.The necking strain and fracture strain of DP780 high strength steel sheet w ere determined from the strain field and strain history results.