The objectives of this research were to find out pronunciation mistakes of rarely-used English vocabulary and identify the factors that affect students’ mispronunciation by focusing the mistakes of segmental features. The researcher used survey as the research method. This study taken place in English Language Teaching Department, Universitas Muhammadiyah Purwokerto. The population of this research was 8th semester students. Most of participants used Javanese Language as their daily language and have such of different mother tongue like Javanese Language (Banyumasan), Indonesian Language, and Sundanese Language. The data was collected through oral test and interview. The findings of this research are: 1) There were such mispronunciations in every word of instrument such as mispronounced from vowels /ə/, /e/, /ɜ/, /i/, /ɒ/, in diphthongs were /ei/, /əʊ/, /ai/ and mispronunciation from consonants /ʃ/, /t/, /st/, /z/. 2) The factors affecting students’ pronunciation are mother tongue, exposure, everyday language, educational background, phonetics ability and motivation.