Abstract: The current study has aimed to explore the effect of dynamic and non-dynamic assessment on acquisition of apology speech act among Iranian EFL learners. One-hundred forty-three male and female intermediate Iranian learners, who were selected randomly, were assigned to high and low groups based on their score in OPT. Then, each group was divided into two DA and NDA groups randomly. Both groups participated in pretest and posttest, six treatment tests during fifteen sessions, also an instruction part about different apology strategies based on Olshatain and Cohen (1983). DA groups, the experimental ones, received different types of mediation in tests based on Lantolf and Poehner's (2011) scale and their ZPDs. NDA groups, the control ones, were assessed in traditional method. T-test was used to analyze the data, which revealed that DA group significantly outperformed NDA group. The main result of this study showed that dynamic assessment helps learners to improve their apology speech act acquisition. The findings of this study provide insights to assessment of pragmatics.