A TOEFL certificate with a certain score becomes a regulation to graduate from many universities in Indonesia. However, university students face many problems in achieving a certain TOEFL score that has been decided by their department. In order to obtain the required score, students often have to have a good preparation before they take the TOEFL test. The objectives of this research are to know the students’ TOEFL score achievement and to investigate the effect of joining TOEFL preparation course toward students’ TOEFL score. This study was designed as quantitative research with cause effect design. This research population was the whole students who joined a TOEFL preparation course at the Language Training Center (LTC) of Universitas Muhammadiyah Yogyakarta (UMY). The researcher used purposive sampling and 170 students become the research’s sample. The students’ TOEFL score in pre-test and post-test were used as the data in this research. To answer the research questions, the researcher used descriptive statistic and inferential statistic in the data analysis method. The results revealed that, in average, students’ level of TOEFL proficiency in pre-test and post-test were in intermediate, which were 386.62 and 415.45. The increasing score was 28.84. Besides, the significant level of t-test showed 0.000 and the effect size was 0.72 which indicated that joining the TOEFL preparation course was proved to give significant effect to improve students’ TOEFL score. Based on the results, joining the TOEFL preparation course was effective to improve students’ TOEFL score.