The purpose of the study is to reveal the effect of cooperative learning on language skills in an English course. The study was carried out for 5 weeks with 66 students studying at the 10th grade at an Anatolian high school in the district of Karade niz Eregli in Zonguldak during the fall term of the 2015-2016 academic year. The design of the study was 'nonequivalent control groups pre-test post-test' which is one of quasi-experimental designs. Data were gathered using an achievement test measuring students' vocabulary knowledge, grammar knowledge, reading comprehension and listening skills. Results showed that cooperative learning had a larger effect on vocabulary knowledge, grammar, listening and reading skills compared to traditional method.