Different scholars suggest that students’ mathematics performance at any level is worsening from time to time. For example, the four National Learning Assessment reports (1992, 1996, 2000 & 2004) of grade eight students mathematics performance in the Amhara region disclosed that it is very far below from the expected mean, which is 50 %, and going down from time to time. Furthermore, by 2016, only approximately 33% of the students who took grade 10 national exam in the region have got a passing mark. The objectives of the study were to investigate secondary school students’ mathematics performance and perceptions resulting from engagement in cooperative problem-solving approach. Both quantitative and qualitative research approaches were utilized in this study. Quasiexperimental design was used since the study was conducted in a classroom setting and it was not possible to assign participants (students) randomly to groups. The intact groups were used for the control and experimental groups. The actual classroom teaching in the experimental group was conducted by one of the teachers in the school using CPS approach. The experiment took five weeks (25 hours). In addition, questionnaire, interview and focus group discussions were used as data collection instruments. The statistical techniques used were independent samples t-test, paired samples test, Analysis of covariance (ANCOVA) test and one sample t-test. In comparing mathematics performance, the independent samples test results indicated that there was a significant difference between the achievement mean scores of experimental and control groups, ( t(118) = -49.56 and p = .000), which is significant at the alpha level of .05 in favor of the experimental group. The difference, effect size (d = 9.05), is much larger than typical using Cohen’s (1988) guidelines. After cooperative problem-solving is employed in learning ‘relations and functions’ in grade 11, cooperative problem-solving approach improves the secondary school students’ mathematics performance, the students’ higher levels of cognitive learning and their perceptions to CPS. Students preferred to be engaged in non-routine problems in groups. Keywords: Cooperative Problem-solving, Mathematics Performance, Quasi-experimental, Perception