Assessment of writing skill is generally believed to be judged by a rater subjectively and qualitatively or by using analytic scoring rubrics which can potentially result in somehow not very reliable assessment. It seems that an evaluation of writing based on a model can result in a valid and reliable writing assessment. To achieve such an objective, this study firstly aimed to develop an assessment model based on Activity Theory (AT), i. e., Activity Theory-Based Assessment Model (ATBAM), and then to employ it in the assessment of writing performances of Iranian language learners in a private language college. And finally, to achieve the concurrent validity of ATBAM, its results were compared with those of a traditional approach. Three groups of participants took part in this study: a group of upper intermediate English learners (N=29) who submitted one writing sample per week in four successive weeks, teachers (N=6) who provided learners with feedback and assigned holistic scores and course supervisors (N=2) who reassessed the writing samples on the basis of an analytic rubric of writing assessment. The results showed that using ATBAM in writing assessment results in an exploration of not only learners’ but also teachers’ engagement in the development of learners’ writing ability. The role of teachers’ feedback and teachers’ and learners’ social interaction in the development of learners’ writing ability could potentially provide comprehensive, fair, reliable, and valid scores in this model.
