This study examined Taiwanese ESL learners' perception and production of English interdental fricatives (IFs) with respect to three variables: perception errors, production errors, and three ranges of length of residence (LOR) in the US (LOR < 1 year; LOR= 3-4 years; LOR ; at least 7 years). Through the investigation, several relationships among perception, production errors, and LOR were revealed. In addition, the types and patterns of the pronunciation errors as well as the phonological processes involved in the mispronunciations were investigated. The instrument included both perception and production tasks, as well as a demographic questionnaire. More specifically, the production task contains two syllable positions: word initial and word final; two sources: word list and text. The data were analyzed through auditory and spectrographic devices with an involvement of an inter-rater agreement. The results showed that there was a positive relationship between perception and production errors for each of the three groups. Namely, the more perception errors made by the Taiwanese ESL learners, the more production errors occurred. The correlation increases with each group, i.e., with LOR. The relationship between LOR and perception and production errors were significantly negative as the longer the learners had stayed in the US, the fewer errors in perception and production were made and vice versa. A descriptive analysis revealed that the initial theta and the final eth are the most difficult sounds for the Taiwanese learners to perceive. In addition, both voiceless and voiced IFs were found to be difficult to pronounce in the final position. Regarding the source where the target sounds were located, the results indicated that reading text (rather than a word list) caused more pronunciation errors for most of the learners except the group with LOR at least 7 years, which had a better performance in the production task compared with the other two groups, regardless of which source the target sounds were located. From the overall findings, ESL teachers are encouraged to apply multiple teaching tools and to provide feedback with effective correction techniques to learners in order to adjust and overcome their pronunciation errors.