The study aimed to identify the problems encountered by Grade 1 Mathematics teachers in teaching Mathematics using the Mother Tongue as a medium of instruction in the schools of Division of Dumaguete City and their relationship to class academic performance of the pupils. The survey was descriptive and correlational in nature. It utilized frequency distribution, percentage, mean, weighted mean, and Pearson Product Moment Correlation Coefficient. Majority of the teachers are non-master’s degree holders and have been in the teaching profession for a longer time. Most of them attended the indicated seminar-workshops related to the K-12 instruction. The teachers encountered a “serious” problem in the translation of Mathematical concepts or language in the Mother tongue and had a “moderate” problem in the utilization of effective questioning skill and in giving of exercises and activities. Moreover, they faced a “slight” problem in the utilization of effective reacting technique. The findings disclosed that the class academic performance of the pupils is in the “approaching proficiency” level. There is a “slight” negative relationship between the extent of problems encountered by the teachers in the translation of the Mathematical concepts and the class academic performance of the pupils. There is also a “slight” negative relationship between the number of relevant trainings attended by the teachers and the following variables: utilization of effective questioning skill, utilization of effective reacting technique and giving exercises and activities. Moreover, the study showed that there is a “slight” negative relationship between the educational attainment of the teachers and the following variables: utilization of effective reacting technique and giving of exercises and activities. It is, therefore, recommended that: Teachers upgrade their skills in managing classroom instruction by pursuing further studies and attend more training focusing on the translation of the content of the Mathematics competencies in the Mother tongue. Teachers, in coordination with the School Head, Public Schools District Supervisor, and Education Program Supervisor for Mathematics, formulate varied exercises activities in the Mother Tongue so as to enhance learning.