The present study investigates the impact of self-assessment (SA) training applied as a writing task and a dynamic assessment on English language learners' writing ability and explores the changes in their language proficiency level. This quasi-experimental research on two homogenised essay writing classes of 60 Iranian university students at Islamic Azad University in a whole term indicates a significant effect of the use of SA in developing their writing skill and proficiency levels. The experimental group and their teacher are also interviewed about their experiences at the end of the study to investigate their attitudes towards SA.