There is little research that relates formative assessment to writing, particularly in a foreign-language context. Furthermore, few studies have investigated processes of change when teachers and students use a tool to enhance their skills in formative assessment practices. This article reports on an intervention study where portfolio was used to enhance middle-school teachers’ and students’ perceptions and practices of formative assessment of writing in English as a foreign language (EFL) classes. As such, the study provides new knowledge. The findings suggest that students appreciate the processes of change, despite initial resistance due to the use of novel assessment practices. The teachers see the value of including self- and peer assessment and text revision in assessing writing. They view the processes of change as part of their professional development. Computer issues and few teaching hours proved to be impeding factors in changing their assessment practices.