The present study was undertaken to explore the practicality and the effect of dynamic assessment on L2 writing ability of Iranian English as a foreign language (EFL) learners. To this end, 17 EFL learners divided into two groups, participated in this article. Then, dynamic assessment procedure was done in three steps, topic-choice, idea-generation, and macro-revising. All these steps ran with the mediation of the teacher and also the learners together. The results of the study indicated that dynamic assessment significantly influenced participants' scores, enhanced their writing ability, and illustrated that experimental group's dynamic assessment scores were generally higher than the control group's scores. The results of the learners' interview assured that dynamic assessment could improve the learners' EFL process writing and their writing confidence. It also elevated their motivation in their writing ability.