abstract the purpose of this study was to find out the effect of applying the principles of group-dynamic assessment (g-da) on learning of conditional structures in english by iranian efl learners at the intermediate level, which according to the formal educational system in iran, includes students who are in their second year of studying in high schools of koohdasht city. this study was a qualitative and quantitative study and data were collected using pre and posttest, samples were 40 students and 10 students for the purpose of qualitative study were interviewed. students were divided into two experimental groups of the concurrent g-da and cumulative g-da and each group underwent five extra classes sessions of teaching using principles of each approach. data was analyzed using spss 20.0 in the two stages of descriptive (frequency, percentage…) and inferential study (one sample t-test and independent sample t-test). result showed that with 95% confident teaching 5 sessions of g-da influences on knowledge of language skills (in this study conditional structures). also, the concurrent g-da approach significantly was better than cumulative g-da in learning conditional structures. in addition result of interview was in line with the result of experimental study indicating that group dynamic assessment approach reinforces students’ language learning and approved this idea that concurrent g-da is prior than cumulative g-da.