The purposes of the present study were to examine the language errors in a writing of English major students in a Thai university and to explore the sources of the errors. This study focused mainly on sentences because the researcher found that errors in Thai EFL students' sentence construction may lead to miscommunication. 104 pieces of writing written by 26 second-year English major students who enrolled in the Writing II course were collected and analyzed. Results showed that the most frequently committed errors were punctuation, articles, subject-verb agreement, spelling, capitalization, and fragment, respectively. Interlingual interference, intralingual interference, limited knowledge of English grammar and vocabulary, and carelessness of the students were found to be the major sources of the errors. It is suggested that intensive knowledge of English grammar and vocabulary be taught to Thai EFL students. Moreover, the negative transfer of students' first language should be taken into account in English writing classes. This finding also implies that explicit feedback on students' writing errors is genuinely needed.