Teaching mathematics in high schools not only provides knowledge to solve problems or apply them into practice but also trains the students' ability to think mathematically. Therefore, in order to promote the students' mathematical thinking skills, teachers need to involve students in learning activities, help them train and develop their thinking. The purpose of this study was to enhance creative and critical thinking skills for 10th-grade students through teaching inequality topics. The experimental class had 41 students and was taught with activities of proving an inequality to enhance critical and creative thinking skills; while the control class consisted of 35 students and they approached conventionally. The qualitative method was used to evaluate students' creative and critical thinking skills for two types of exercises, namely, proving an inequality and finding the maximum value of a function. The findings showed that students in the experimental class showed a better ability to prove inequalities than students in the control class, and they also presented more solutions for a problem proving the inequality.