This study aimed to find out whether comic strips improve the students’ mastery on idiomatic expressions. The research used a pre-experimental type with one-group pre-test and post-test design. The population was the second-grade students of the Senior Islamic Boarding School in Lombok. The researcher chose one class which consisted 16 students as the sample. The data was analyzed by using t-test. The data analysis revealed that the use of comic strips on students' idiomatic expression mastery was successful. The increase in the pre-test to post-test score of students’ average scores is an indication that the students have really understood and done well in all the questions assigned to them. The treatment given using comic strip media is very helpful for students in answering the questions given. Comic strips help students in overcoming errors in interpreting the idiom. The research analysis of students’ scores on the pre-test and post-test scores, it shows that the value of the t-test result is 0.000 0.05, therefore it can be concluded that the alternative hypothesis (Ha) is accepted and the null hypothesis (Ho) is rejected.