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 macrorevising. 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.Consequently, the present study attempts to find answers to the following questions:Research Question 1 (RQ1): Is dynamic assessment useful for foreign language learners' process writing? Research Question 2 (RQ2): What is the effect of dynamic assessment if it is a useful way for assessing EFL learners' process writing? Research Question 3 (RQ3): How do the participants of the study view dynamic assessment process? Research Question 4 (RQ4): What is the effect of dynamic assessment on the topic-choice stage of writing development? Research Question 5 (RQ5): What is the effect of dynamic assessment on the idea-generation stage of writing development? Research Question 6 (RQ6): What is the effect of dynamic assessment on the macro-revising stage of writing development?