This study investigated the effect of dynamic Assessment (DA) on the acquisition of L2 grammar by EFL learners. The focus was on teaching Conditional Type II, or Unreal Conditional, which is a difficult structure for language learners to acquire. To this end, two intact classes of intermediate EFL learners, each consisting of 23 male students were assigned to control and experimental groups. They were pre-tested and post-tested (immediately and delayed post-test) using a written grammaticality judgment task. Multivariate Analysis of Covariance (ANCOVA) was run to analyze the data. The results revealed that the experimental group significantly outperformed the control group (p<0.05) on both immediate and delayed post-tests. The findings point to the advantage of implementation of DA in L2 grammar instruction in classroom contexts.