This article is written based on the research done by the writer as an English teacher at Wijaya Kusuma Senior High School Surakarta. There are two aims on this research. They are : (1) to test the hypothesis that group discussion can improve the students’ speaking skill of the first students of Wijaya Kusuma Senior High School, and (2) to describe how group discussion improves students’ speaking skill. The writer used classroom action research as the method in conducting this research. And the technique used is group discussion. Action research occurs through a dynamic and complementary process, which consists of four essential moments; of planning, action, observation, and reflection. The data are obtained from the observation and research. There are two cycles on the research done by the writer. Each cycle consisted of three meetings. Every meeting covered greeting, main activity and closing. The research shows improvements of their scoring and performance. On pre-test, the mean of their score was 3.7, but in the post-test 1, their mean improved to be 5,6667. And its improvement was continued on the post-test 2, where their mean become 7,6667. The result shows that there are some improvements on the students’ speaking skill quantitatively and qualitatively.Keywords: action research, speaking skill, group discussion