This study was a mixed-method design to explore the efficiency of alternative assessments in boosting language performance. To achieve this objective, 60 Iranian Language learners participated in the study. The participants were advanced students of three intact classes in an Iranian online language center in Sirjan, Kerman (A province in South East Iran). The study focused on the effect of Self- and Peer-Assessment versus Teacher-Assessment on enhancing the Writing Skill of Iranian EFL Learners in the online context. To conduct the quantitative (trueexperimental) phase, the three classes were randomly assigned to two experimental groups (N= 19, N=19) and a control group (N=22). The classes were randomly chosen to be studied as self-, peer- and teacher-assessment groups. To answer the research questions of the quantitative phase of the study, a mixed design ANCOVA with one within-subject variable (writing performance), one between-subject variable (group), and two covariates (OOPT and Pre-test scores) were launched. According to the results, the self-assessment group in both ratings obtained higher posttest scores compared with peer- and teacher-assessment groups. All groups showed significant gains from pre-test scores to post-test scores. However, the self-assessment group showed higher gains compared to the other two groups. The results of the qualitative (exploratory, descriptive, and interpretive) phase of the study revealed that nearly all the students believed that selfassessment processes in the light of the teacher’s guidance led to their improvement in writing. The results indicated that students did better in realizing their problems and improving their writing when they received their peers’ feedback