Physics educators around the world often need reliable diagnostic materials to measure students' understanding of physics concept in high school. The purpose of this study is to evaluate a new diagnostic tool on High School Optics concept. Test of Conceptual Understanding on High School Optics (TOCUSO) consists of 25 conceptual items that measures fundamental topics learned in high school curriculum. TOCUSO was designed with the use of various Optics textbooks, Turkish University exam questions, and questions generated by the author. TOCUSO items were structured as multiple-choice questions. 12 physics faculty members at the school of science and education validated questionnaire items. SPSS program was utilized to calculate reliability values of the test. KR-20 value was found around 0.73, which shows TOCUSO is a reliable measurement questionnaire. TOCUSO was applied to 183 high school students enrolled in 10th grade at the time of data collection process in Kayseri providence in Turkey.