The research was aimed to find out whether or not using debate significantly improved the students' critical thinking and speaking skill achievements and how much debate contributed to each aspect of critical thinking and speaking skill. A quasi-experimental study of non-equivalent pretest- posttest control group design was used in this research. The findings showed that (1) there was a significant improvement in critical thinking and speaking skill, (2) there was also a significant mean difference between the experimental and control groups, (3) there was high contribution of the debate toward the whole aspects of critical thinking (0.821 or 82.1%). Partially, the contribution of each aspect of Critical Thinking (CT) toward critical thinking (total) achievement was as follows: context was 32.3%, issue was 26.2%, implication was 20.1%, and assumption was 6.6%. On the other side, there was high contribution of the debate toward the whole aspects of Speaking Skill (0.961 or 96.1%). Partially, the contribution of each aspect of Speaking Skill (SS) toward Speaking Skill (total) achievement was as follows: fluency was 67.4%, grammar was 13.7%, pronunciation was 8.3%, comprehension was 5.4%, and vocabulary was 1.4%.