Abstract Salwa Chaira, 2014. Interference of First Language in Pronunciation of English Segmental Sounds (A Case Study in Darul Ulum Islamic Boarding School Banda Aceh). Thesis. Darussalam, Banda Aceh. Advisors: (I) Abdul Gani Asyik, M.A., Ph.D, (II) Dr. Sofyan A. Gani, M.A. Key words : interference, consonant and vowel sounds, pronunciation, first language (L1) This research is aimed to investigate the interference that occurs in Darul Ulum students’ English speaking since the reality shows that the students mispronounce some consonant and vowel sounds of English. This phenomenon occurs because the students are required to constantly apply English in any circumstances. When they are unfamiliar and attempt to pronounce new words without prior knowledge of how to sound them correctly, they may produce errors even fossilize them. Thus, the teachers are recommended to halt the errors production and direct them to correct pronunciation. This research focuses on investigating the interlingual errors the students produced as the result of the interference and finding solutions of how to avoid the interlingual errors through the methods applied by their teachers. To reach such purpose, the writer applied a subsequent process method by collecting the data on the field, selecting the required samples, classifying into the sound classification, comparing the taken samples from English pronunciation using phonetic transcription, analyzing the data using phonological theories. Here are the mispronounced sounds resulted from the interference; [ph], [th], [kh], [f] for grapheme “ph”, [v], [θ], [ð], [z] for grapheme “s”, [ʃ], [ks] for grapheme “x”, [iː], [uː], [æ], and [e]. Therefore, teachers are recommended to apply Audiolingual Method, Phonetic Method, pronunciation drill, Behaviorist Learning Theories (sound imitating), and Phonic-based Approach to improve the pronunciation of the consonant and vowel sounds. Through this language research, the teachers will definitely recognize of how to sound all English consonant and vowel sounds correctly. Therefore, they are recommended to implement the given methods to have their students correctly pronounce the sounds in order to avoid the interlingual errors caused by the interference of their L1.