This study aimed to describe the effectiveness of guided inquiry based learning to train critical thinking skills on ecosystem topic in the high school level. The trial test in this study involved 10 th graders of SMA Negeri 1 Sumenep in the second semester of academic year 2017/2018 by implementing the modified guided inquiry based learning devices. The research design was that used in this study was One-Group Pretest-Posttest Design. Data of students' critical thinking skills were obtained from students' answers to the six critical thinking skills instruments. The results obtained from this study is in the form of critical thinking skills data of students viewed from the gain score and students' mastery learning outcomes. The gain score obtained by the students has an average of 0.74 which categorized as high. Moreover, the percentages of students' mastery learning data showed that 100% of students succeeded after being given the treatment. Based on these data, it can be concluded that guided inquiry based learning enables students to practice their critical thinking skills on ecosystem topic.