The purpose of this quasi-experimental study was to investigate the effects of cooperative learning on EFL junior high school learners’ language learning, motivation toward learning English as a foreign language, and the high- and low-achievers’ academic achievements in a heterogeneous language proficiency group. A pretest-posttest group research design was used. The sample population was from two classes of the first year junior high school students in a rural town in central Taiwan. There were totally 70 students involved in this study. The experimental group was taught in cooperative learning for one semester with the methods of Three-Step-Interview, Learning Together, Talk Pair, Inside-Outside Circle, and Student-Teams-Achievement Division. The control group was taught in the traditional method of Grammar Translation with some of the Audio-Lingual approach.