Significant consequences are results of mere decision on determining the language of instruction specifically in the early years of education. It means that rightful language choice leads to educational success. Contrariwise, an inappropriate selection of a language proves to be detrimental in the taking place of learning. With the recent language policy shift from English to Mother tongue (MT), in the context of the Philippines, as medium of instruction in the teaching of mathematics in early grades, this study examined how language proficiency in the MT relates to mathematics achievement. Study participants include 71 grade 2 students aged 6-8. The findings show that the respondents' mean achievement in mathematics and their mean proficiency in the MT are both described as 'advanced'. Moreover, it was revealed that there is a very strong positive correlation between the respondents' achievement in mathematics and proficiency in the MT.