This study attempted to determine the effect of teaching using Appreciative Inquiry approach in teaching trigonometry. Sixty first year students enrolled at John B. Lacson Foundation Maritime University-Arevalo for the second semester of 2008-2009 were randomly assigned to AI and Non-AI groups. A group composed of thirty students was subjected to teaching using Appreciative Inquiry, while another group of thirty students who were match-paired with the first group of students using mathematics I grades composed the group that was not exposed to teaching using Appreciative Inquiry. This study made use of the pretest-posttest control group quasi-experimental design and qualitative inquiry method. Sources of data were a 30-item mathematics performance test, pre-treatment and post-treatment appreciation questions, appreciative interview questions, and focused group discussions. Mean, standard deviation, and t-tests for dependent and independent samples tested at .05 level of significance were used. Results showed that both the two groups (AI and Non-AI) were comparable at the start of the study. No significant difference was noted in the mean gain performance scores of the two groups although the grouped exposed to teaching using Appreciative Inquiry had a higher mean gain as compared to the other group. During the pre-treatment, students find mathematics as important and useful but it is challenging and difficult subject for them. In the post-treatment, students find mathematics as challenging and very interesting. Moreover, they find it fun, enjoyable and easier to learn now. During the AI process, it was revealed that students' great/peak learning experiences happened when: teachers were at their best and effective strategies were employed, teachers' positive nature was felt and observed, teachers' preparation was revealed in answering students' questions and fun classes were conducted by these teachers. Identified core values/factors for peak learning experiences were: friendships, where friends mentor friends; when help, support, and cooperation were present among the students and the teacher; and, when there was teacher's sense of humor in class.