The current study explored the effect of portfolio assessment on the improvement of the Iranian students' writing. Additionally, students' attitudes towards using portfolio were investigated. Participants of the study were 46 pre-intermediate Iranian male and female university students who were chosen from a larger group of 70 students. They were divided randomly into experimental and control groups with equal number of students of 23 in each group. All the students in experimental group wrote five essays based on the topics in their course book. To evaluate the students' writings, word choice, ideas, sentence fluency, voice, and organization were checked by two experienced raters. In contrast, the students in control group wrote five essays and they didn't receive any feedback from their teacher and also they didn't have any opportunity to revise their writing papers. The findings of the study revealed that portfolio assessment had a positive effect on the overall writing ability among the Iranian university students.