Undergraduate students' experience of assessment in universities is usually of summative assessment which provides only limited information to help students improve their performance. By contrast, formative assessment is informative and forward-looking, possessing the leverage to inform students of their day-to-day progress and inform teachers of how to better tailor their instruction to students' immediate learning needs. Despite these potentials, studies carried out on the use of formative assessment in English as a foreign language (EFL) contexts are somehow rare. The current study reports on incorporating formative assessment in an L2 writing course in Iran. The analysis of data from pre- and post-study writing tasks, pre- and post-study questionnaires, and semi-structured interviews revealed that first-year undergraduate students were offered opportunities to improve various aspects of their writing and to develop positive attitudes toward writing as well as formative assessment. However, the students reported several challenges that could have implications for the further implementation of formative assessment in similar contexts.