The present study attempted to investigate the impact of electronic portfolio assessment on performance of Iranian EFL learners’ writing. To do so, 30 advanced EFL learners who participated in a TOEFL preparation course were selected as the participants of the study. After administrating a truncated version of TOEFL proficiency test, they were randomly assigned to control and experimental groups. The experimental group was given a treatment including electronic portfolio assessment, while the control group was given a placebo. To collect the required data, two instruments (a writing pre-test and a writing post-test) were administered to both groups during the experimentation. Subsequently, the learners’ scores were collected and the results were statistically analyzed. Inter-rater reliability, matched t-test, and independent t-test were calculated. The findings revealed that the participants of the experimental group outperformed those of the control group and thus it was concluded that electronic portfolio assessment can improve the writing ability and can be considered as a motivating assessment strategy.