Grammar-based instruction is one approach to teaching and learning languages, particularly second or foreign languages. Grammar is not taught directly or literally; rather, it is used to facilitate language acquisition through grammatical knowledge, which is applied using a number of ways. This overview demonstrates how grammar-based instruction influences junior high school students English language skills. Thus, teachers need to employ some strategies like inductive approach and deductive approach also activities in the classroom to better assist the learners in enhancing their linguistic competence. The purpose of this study is to determine the effectiveness of using grammar-based instruction in enhancing students linguistic competence. This study employed a one group pre-test and post-test experiment to collect data in numbers for statistical assessment. In addition, a cause-and-effect relationship between grammar-based instruction and the linguistic competence is the goal of this study. The respondents of this study are the forty (40) Grade 8 junior high school students of Lusacan National High School. The study was conducted during the third grading period of academic year 2023-2024. Moreover, the result revealed that there is no significant difference between the pre-test and post-test scores of the respondents on their linguistic competence in terms of lexical, morphological and syntactical before and after using grammar-based instruction. Thus, the null hypothesis is rejected.