Writing is usually considered as the most difficult skill to master for the students who are learning English. It has many complex processes such as prewriting, drafting, editing, and revising. Writing is often influenced by the constraint of genre. One of the genres is descriptive text. In fact, the researcher found some problems in conveying English materials to the students. An effort was done to solve the problem by using poster. This study aims to find out the students’ performance before and after the treatment using poster. Moreover, the researcher investigated the effectiveness of using poster in teaching descriptive text. The research used quasi experimental. The participants of this research were 20 students at eighth grades of Junior High School in Yogyakarta. The data were collected using tests including pretest and posttest. The result of this research showed that there was a significant difference between students’ performance before and after the treatment by using poster. It could be seen from the mean score of the pretest which was in the poor level (5.24) and mean score of posttest was in fair level (6.64). The finding also showed that number of effect size was 0.90 which means that the result was categorized to have a big effect. The result showed that T-value was higher than T-table, in which Tvalue score was 9.471 while T-table score was 1.729. It indicated that the alternative hypothesis was accepted. In conclusion, posters had a significant effect in improving students’ writing skill in descriptive text.