© 2021 BOLEMA Departamento de Matematica. All rights reserved.In this study, we conducted peer instruction (PI) activities to promote student participation in the learning process and test the hypothesis that PI improves student achievement. Two ninth-grade classes were randomly assigned as treatment and control groups. Pre-Test and post-Test data were obtained for measuring mathematics achievement in trigonometry. Data were analyzed using analysis of covariance procedures with an alpha significance level of 0.05. Results indicated no significant effects of peer instruction on achievement. This study implies that more robust studies are needed to reveal the real effect of PI.