This study evaluated the effect of Mother-Tongue language (Waray-Waray) to the Elementary Grade Pupils (EGP) in Mathematics. An experimental research design using pretest-posttest equivalent group design was utilized in this study. A teacher-made test was utilized to measure the students’ achievement in Mathematics covering the topics Operations of Whole Numbers, Operations of Decimal Numbers, and Laws of Exponent. The results revealed that respondent’s Mathematics achievement rose high for experimental group and very high for control after the posttest, with the control group more homogeneous in term of their scores in the posttest. Teaching Mathematics in English allowed the respondents to perform better than the respondents under the Mother-Tongue (Waray-Waray) mediated Mathematics instruction. Since Mother-Tongue in Samar College is English, pupils may encounter Waray-Waray mediated Mathematics instruction unfamiliar. Therefore, this recommended that English as the Mother-Tongue in Samar College Elementary Department must be utilized in Mathematics enhance the learner’s understanding of the concept and teachers should explain the lessons in the learners’ language, particularly when the teacher notices that the students are having difficulty in understanding the topic.