This study aims to find out the most mispronounced and describe the factors of pronunciation errors of triphthong sounds produced by the EFL students of IKIP PGRI PGRI Pontianak. The total sample was 34 EFL students and taken
by purposive sampling technique. This research design used the descriptive method. This research used an oral pronunciation test and interview. The oral pronunciation test was distributed to 34 students in one class of class in the third-semester students of the English Education Department in academic year 2021/2022 at IKIP PGRI Pontianak. The interview was distributed to 4 students who have the highest and lowest errors in the oral pronunciation test. The process of the data analysis was elaborated based
on the Miles and Huberman theory which are data reduction, data display and conclusion drawing.
The research findings of this study revealed that the most mispronounced of English triphthong sound by the EFL students is /aɪә/ with 26% errors, followed by /aʊә/ with 24% errors, /eɪә/ with 18% errors, /әʊә/ with 18% errors,
and lastly /ɔɪә/ with 14% errors. This study has also shown that factors that affect pronunciation errors are the students' native language, the amount of exposure to
English, students' phonetic ability, and lastly, students' motivation and concern for good pronunciation. This result is almost similar to Kenworthy's theory except for the age factor.

Keywords: EFL Students, Error Analysis, Pronunciation, Triphthong