Errors in writing are unavoidable while students are trying to develop their writing skills. There have been several studies on identifying writing problems or errors in students' writing. It is believed that identifying students' written tasks is an effective tool to explore the difficulties involved in learning language. This helps teachers' awareness of the serious problems which occur in students' writing and allows them to pay closer attention to their errors. The aim of the present research study is to pinpoint writing errors in English abstracts written by Thai undergraduate students. Forty abstracts of research projects were collected and analysed. The error analysis was conducted at the sentence level, word level, and mechanics aspect. The five most frequent error types ranking from the most frequent to least frequent were word choice, preposition, sentence construction, singular or plural forms and quotation marks. The findings of the present study have shed light on the students' writing ability and give an insight into what the problems students face when writing their abstracts. Also, the errors found in the abstracts in the present study have pedagogical implications concerning English language learning, particularly with writing courses. The findings will be helpful for teachers to develop teaching materials to assist their students from committing errors when writing English abstracts and to improve academic writing skills.