Purpose: In class discussion method can be used while studying Mathematics as it is intended to be a free give and take between teacher and students and among students on the current topic of concern in the course. It is characterized by probing questions from the teacher designed to elicit student interpretations, opinions, and questions. The purporse of study is to assess the effect of group discussion method on the performance of Mathematics students in public day Schools in Rwanda” A case of Karongi District’.
 Materials and Methods: The study used descriptive research design. Stratified sampling was used to determine a sample of 6 schools among other public secondary schools in Bwishyura sector in Karongi District. The head teachers of the sampled schools were purposively selected to participate in the study. Simple random sampling technique was further applied to obtain Mathematics teachers and students of senior 4,5 and 6, making 783 target students. The study used a sample of 230 respondents from the sampled schools. The collected data was analyzed using descriptive statistics, correlation, and regression analysis through the statistical package for social science (SPSS) version 21. The data collected were analyzed using multiple linear regression analysis. The probability value of the regression estimate was used in testing the hypotheses of the study.
 Findings: The result of the regression analysis indicates that Demonstration Method has a positive effect on Performance of Mathematics students in public secondary Schools in Karongi District and the relationship is statistically significant (p=>0.05). Results showed significant positive relationship between discussion method and the performance of Mathematics students (r = 0.860; p= 0.000). Multiple linear regression analysis showed that discussion method contributed to 72% of variation on the performance, hence plays a vital role in performance of Mathematics students in public secondary schools in Karongi District.
 Implications to Theory, Practice and Policy: The study recommended that Mathematics teachers should be trained and encouraged in the use of discussion teaching method and the framework for planning lessons and teaching mathematics in our secondary schools should be on group discussion teaching strategy. Moreover, it was also useful to enable learners to take part in discussion, reflection, feedback and to consolidate learning, clarify understanding and explore ideas and concepts.