The present study sought to investigate the effectiveness of teacher versus learner assessment activities on the Iranian intermediate EFL learners' writing ability. In order to find an answer to the questions of this research, 30 intermediate students from Zaban Iran Language Institute in Rasht, Iran were selected via administering Solutions Placement Test (SPT). Next, they were divided into three groups of 10. Learners were randomly assigned into two experimental groups of peer assessment and self-assessment, and one control group, teacher assessment. A pre-test of writing was then administered before the groups received 8 sessions of treatment through peer, self-, and teacher assessment techniques. After the treatment period, a posttest of writing was administered to all groups. The results of descriptive and inferential analyses revealed that the peer assessment group attained the highest scores on the writing test, and a statistically significant difference among the effects of the teacher assessment, peer assessment, and selfassessment on Iranian Intermediate EFL students' writing ability was reported. Also, the group using teacher assessment technique attained higher scores than the group that used the self-assessment technique. The implications of this study can be considered for EFL teachers, learners, materials developers, and syllabus designers.