Tasks have played a significant role in both early descriptive research and subsequent more theoretically-based research. The purpose of the present study was to measure the effect of collaborative output tasks on improving Iranian EFL learners' lexical collocation. To do so, 60 female students were selected to participate in this experiment, based on their performance on Oxford Placement Test (OPT). They were divided into two equal groups, experimental and control. The participants in the experimental groups were assigned collaborative output tasks involving collocations in order to review them without memorizing or learning by rote. The control group got vocabulary education using the school's standard teaching methods. The posttest was administered to both groups' participants in order to assess the students' lexical collocation knowledge. Comparing the posttest of the two groups revealed a significant difference between the score of the experimental groups and that of the control group in the posttests. It was proved that collaborative tasks were effective in enhancing EFL learners' collocation knowledge. Collaborative output tasks are useful tasks in promoting communicative interactions among students and at the same time provide them with a focus on linguistic forms. The present study investigates the effectiveness of output tasks, so it helps the teachers to develop such tasks in their teaching methodology to improve the quality of their teaching.