This study aimed to investigate teachers' perceptions and practices of cooperative learning (CL) in English language classrooms. It was conducted with the participation of 46 teachers (34 females and 12 males) from 10 schools (7 senior and 3 junior secondary schools) in a city in the Mekong delta -- Vietnam. All the participants had used CL in their classroom at least two year before they participated in the study. To find out teachers' perceptions of CL in English language classrooms, the study used questionnaires and interviews. The questionnaire consisting of 50 items was designed based on Likert scale aiming at collecting data about their perceptions and practices to promote CL in their classrooms. The interviews with six of the participants were conducted to obtain more insightful understanding of cooperative learning and how they practiced promoting cooperative learning and difficulties they had when they applied it in their classrooms. The results show that most of the investigated teachers had positive perceptions of applying cooperative learning, especially they had good understanding of it (M=4.3, SD=0.479). Regarding difficulties the teachers had, the results showed that the big class size, the noise and class control losing were claimed to be regular barriers. Besides, they also offered some ways to promote cooperative learning in English classrooms including clearly assigning roles for students, setting the stage for learning and closely monitoring the groups.