Pronunciation of the correct English is important because if mispronunciation occurs, the meaning will not be delivered effectively. However, there are some differences in the sound system between English and Bahasa Indonesia. Therefore, there will be some difficulties for Indonesians to pronounce English diphthong sounds accurately. Therefore, this research aims to find out the mispronunciation of English diphthong sounds made by the 1st semester ELESP students from Sanata Dharma University of academic year 2022/2023 that takes Pronunciation Tutoring Class D. Additionally, the factors influencing them to mispronounce diphthong sounds are also analysed. A theory stated by Roach related to 8 English diphthong sounds and Kenworthy’s theory about factors that influence mispronunciation in producing English diphthong sounds are used. The data is gathered through a pronunciation test and a close ended questionnaire. This research is qualitative research. The method used to gather and analyse the data is a coding technique. The steps are familiarizing and organizing, coding and reducing, interpreting and representing. The result shows that the students mispronounced the words containing diphthongs /ʊə/, /əʊ/, and /ɪə/ the most. Furthermore, the native language interference and the amount of exposure they experience are the main factors influencing their pronunciation.