As an attempt to shed more light on the effectiveness of alternative assessment in second language learning, the current study sought to explore the effects of peer assessment (PA) and collaborative assessment (CA) on the Iranian intermediate EFL learners' writing ability. To fulfil the purpose of this study, 36 Iranian EFL learners studying English at Kadous English Language Institute in Rasht, Iran, were homogenized as intermediate learners based on their performance on Oxford Solutions Placement Test (OSPT). The final pool of qualified candidates was assigned to one control and two experimental groups comprising 12 learners. All the participants sat for a pretest of L2 writing so that their initial level of writing could be appraised and their homogeneity in writing could be determined. The experimental groups received treatment on English writing through utilizing PA and CA strategies. On the other hand, the control group was taught through the conventional method (i.e. teacher assessment, TA). At the end of a twelve-session experiment, a post-test measuring the effectiveness of the treatments and the participants' writing ability was administered. The results of descriptive and inferential analyses revealed a statistically significant difference among CA, PA, and TA strategies. The results also demonstrated that the CA and PA groups outperformed the control group. However, the CA group performed significantly better than the PA group. Based on the findings of this study, language teachers, materials developers, and education authorities can concentrate on employing CA as the main strategy to foster the writing ability of Iranian intermediate EFL learners. However, PA may also be utilized to teach writing where possible.