The purpose of the study is to determine the significant correlation of using the supplementary materials on learners’ self-efficacy and the difference between formative and summative assessment and mathematical skills. Specifically, it aims to determine the level of supplementary materials component and features; learners’ self-efficacy; and mathematical skills. The research also determines the significant difference between learners’ mathematical skills in formative and summative assessment. Moreover, the significant correlation of using supplementary materials on learners’ self-efficacy was also tested. The research design utilizes in this study is descriptive correlational and purposive sampling techniques was used to select the respondents. Eighty-eight (88) learners of Magdalena Integrated National High School were the respondents of this study. A 40-item test and questionnaire was used to collect data. Weighted mean, t-test, and Pearson product-moment correlation coefficient was the statistical tools used. The results indicate a very high level of validity for the supplementary materials in terms of components and features. Learners' self-efficacy was also rated as very high. However, the learners' mathematical skills varied, with comprehension, analytical thinking, and critical thinking being fairly satisfactory during the formative assessment, while problem-solving did not meet expectations. Upon the introduction of the supplementary materials, there was an improvement in mathematical skills in summative assessment being very satisfactory and satisfactory respectively. The study reveals significant differences in learners' mathematical skills between formative and summative assessments. However, most indicators of the components and features of the supplementary material do not have correlation in learners’ self-efficacy. The study revealed the significant difference in learners’ mathematical skill between formative and summative assessment leading to the rejection of null hypothesis. Additionally, the study reveals that there is no significant correlation and negligible relationship between the components and features of the supplementary materials and learners’ self-efficacy leading to the acceptance of null hypothesis. The researcher proposes seminars for teachers focused on creating supplementary materials. Likewise, there is an active monitoring system for parents to track their children’s progress. Furthermore, future researchers may include other variables as part of learners’ self-efficacy.