The purpose of the present study was to investigate the comparative effect of dynamic and negotiated assessment on EFL learners' writing complexity and fluency. To this end, 72 female intermediate EFL participants, selected from a larger group of 103 learners based on their performances on a piloted PET, in Tak language institute in Dezfoul, Iran participated in the present study and received either dynamic assessment, negotiated assessment, or traditional instruction during a term. Both of the experiments were process-oriented; however, in the dynamic assessment, the negotiation was done through teacher's provision of feedback wherein the negotiated assessment group peer-negotiation was encouraged. The participants' writing complexity and fluency were measured both before and after the instruction through essay writing pre-treatment test and posttest in accordance with Larsen-Freeman's (2006) T-Unit protocol. A Multivariate Analysis of Variance (MANOVA) was run on the posttest scores to test the null hypotheses of the study, the results of which indicated that while dynamic assessment was significantly effective in improving writing complexity (p = 0.007 < 0.05), negotiated assessment yielded significantly better results in boosting writing fluency compared to the results obtained from both control (p = 0.000 < 0.05) and dynamic assessment groups (p = 0.042 < 0.05). Nevertheless, dynamic assessment did not show significantly better results in comparison to negotiated assessment in improving writing complexity (p = 0.084 > 0.05). Learners, teachers, and syllabus designers who are engaged in the process of language pedagogy may use these results. Depending on the focus of their learning, i.e., fluency or complexity, they may choose the optimal choice between these two types of assessment.