This research discusses students’ mastery of idiom and it is influenced by on their writing skill of descriptive essay. The objectives of this research are to measure the students’ mastery of idiom at the fourth semester of Pendidikan Bahasa Inggris UIN Walisongo Semarang, to measure the students’ writing skill of descriptive essay at the fourth semester of Pendidikan Bahasa Inggris UIN Walisongo Semarang and to explain the influences of students’ mastery of idiom on their writing skill of descriptive essay at the fourth semester of Pendidikan Bahasa Inggris UIN Walisongo Semarang. This research used quantitative method with pearson correlation product moment formula and simple linear regression to calculate two variables. The measurement of students’ mastery of idiom used idiom test, and the measurement of students’ writing skill used writing descriptive essay test. The frequency of students’ mastery of idiom was in “Good” category, on the interval 14.1 – 17.7, and the mean was 15.48. The students’ writing skill of descriptive essay was in “Good” category, on the interval 19.7 – 24.4, and the mean was 22.06. The significant regression test showed that the score of (sig) was lower than the level (α) 0.05 or 0.002 < 0.05. It means Ha was accepted and Ho was rejected. It could be concluded that there is a significant influence of students’ mastery of idiom on their writing skill of descriptive essay.