The current study investigated the effect of cooperative and competitive learning strategies on the acquisition of English vocabulary development by Iranian EFL intermediate learners. In addition, it explored what type of theses strategies was more effective. In such doing, utilizing an Oxford Placement Test (OPT), 45 out of 77 Iranian EFL intermediate learners from four language institutes in Tehran, Iran, were randomly selected. Then, the selected participants were equally divided into three groups, i.e. a control group and two experimental groups, (N = 15). On experimental group was taught via cooperative learning, and the other experimental group was taught via competitive learning. The obtained results were analyzed via one-way ANOVA and independent sample t-test. The results revealed that both of these strategies were effective in English vocabulary development by Iranian EFL intermediate students. Furthermore, the findings indicated that the performance of the experimental group via cooperative strategy was better than their counterpart in the experimental group whom was taught via competitive strategy.