Creative thinking ability is the individual imagination to solve problems. Low creative thinking ability causes students find difficulties in resolving problems faced in learning. Guided inquiry combined with brainstorming is an alternative solution to improve creative thinking ability, give students freedom to think to solve the problem. Creative thinking ability developed consists of fluency, flexibility, originality and elaboration. The purpose of this research was to improve creative thinking ability of students through guided inquiry combined with brainstorming on water pollution material. This research was a classroom action research, held in May 2016. The subjects were students of class X private high school in Karanganyar, with total of 41 students. The research data were obtained through observation, interviews, documentation, and testing. The main data source of creative thinking was from the result of test which was compiled based on aspects of creative thinking by Munandar. The validity of the data was confirmed through triangulation techniques. The data analysis was done through descriptive analysis techniques. The increase of fluency, flexibility, elaboration, and originality aspect was 10.3%, 8.6%, 11.87% and 14.4%, respectively. Conclusion of the research is that there was an increased creative thinking ability from pre-cycle to the first cycle in the form of an improvement category from less creative to quite creative category. Evaluation study shows that the use of guided inquiry combined with brainstorming effectively improved the ability to creative thinking.