Organization for Economic Co-operation and Development (OECD) (2016) released the results of Programme for International Student Assessment (PISA) 2015 and reported that the students' performance in mathematics of Singapore and Indonesia had significant differences. There is a strong relationship between textbooks used and mathematics performance of the students. If textbooks differ, students will get a different opportunity to learn and the opportunity to learn influences students' achievement (performance). The purpose of this study was to analyze the trigonometry contents and cognitive demand levels in Singaporean and Indonesian mathematics textbooks. The data were primarily qualitative. Horizontal and vertical analyses were used in this study. The result showed that Singaporean textbooks put more emphasis on all the concepts of trigonometry on right-triangle and further trigonometry (sine/cosine rules) while Indonesian textbooks provided more discussions on angle and its concepts, trigonometry on right-triangle, and graph function of trigonometry (which is a lot more difficult than sine/cosine rules). In addition, Singaporean textbooks provided more mathematical questions requiring higher cognitive demand levels while, Indonesian textbooks provided more questions requiring lower levels. The differences of textbooks' contents and required cognitive demand level probably influenced students' mathematics performance in the two countries. It is hoped that the results will inform curriculum designers and/or textbooks' author(s) in Indonesia, Singapore, and other countries as they review and update the mathematics curriculum and/or mathematics textbooks.