In each educational curriculum, trigonometry is an important subject at upper secondary schools and pre university level that apply in many other subjects such as algebra, calculus, geometry and physics. Many of students have serious problem in learning the trigonometric materials because usually mathematics educators transfer the trigonometric concepts to students through traditional methods that encourage students to memorize the trigonometric concepts. The purpose of this qualitative case study is to introduce the Lesson Study as a new teaching method based on problem solving approach in order to increase the performance of teachers in teaching trigonometry. In this study, a group of three mathematics teachers from an international pre-university centre in Malaysia and the researcher contributed in preparing a research lesson on the topic of the maximum and minimum values of a trigonometric function. Also, this research lesson improved in an existing class containing 10 students. Data collected through observations of discussion meetings and analyzed descriptively. In this research lesson, the researcher discussed teaching the maximum and minimum values of a trigonometric function through variety of solutions and likely misconceptions among students. Maybe this article helps mathematics educators to have better performance in teaching trigonometry through Lesson Study based on problem solving approach.