Writing is the most difficult skill in English, so most EFL students tend to make errors in writing. In assisting the learners to successfully acquire writing skill, the analysis of errors and the understanding of their sources are necessary. This study attempts to explore the major sources of errors occurred in the writing of EFL students. It also investigates the types of errors and the linguistic level that presents the most errors in their writing. Forty narrative essays composed by Thai university students were collected and analysed. Results showed that the mostly frequent types of errors were translated words from Thai, word choice, verb tense, preposition, and comma. The errors derived from two sources: interlingual and intralingual. Interlingual or native language interference was found to be the dominant source of errors. This study provides pedagogical implications for EFL instructors. Instructors should pay attention to the influence of learners' native language.