The study used the developmental research design in developing and validating the module in mother tongue-based instruction in Mathematics 1. There were two groups of validators for this study. The first group of respondents were fifteen (15) experts who were grade 1 teachers from Magalang South District. They identified the essential and non-essential competencies from the Curriculum Guide in Mathematics I under the K+12 Program of the Department of Education (DepEd). The second group consisted of eight (8) experts that were composed of one (1) Schools District Supervisor, one (1) principal, one (1) School District Mathematics Coordinator. two (2) Master Teachers, and three (3) School Math Leaders who validated the module in terms of content, format/design and pedagogy. The study used the Evaluation Rating Sheet for DE and ODeL Modules and Evaluation Instrument for Topics to be Included in the DE/ODeL Module that were adopted from the College of Education of Pampanga State Agricultural University. The study had three stages such as planning, development, and validation. Based on the results that were gathered, it was suggested that the proposed module in mother tonguebased instruction in Mathematics be used as one of the materials of Grade I pupils in the learning process. Keywords: mother tongue-based instruction , development, validation.