This study adopted a mixed method integrating corpus techniques and error analysis to investigate Vietnamese EFL learners' use of conjunction in English writing. The participants were ten-graders of a Hanoi-based high school. The results showed that additive and causal conjunctions were the most frequently used types, and the students encountered challenges mostly in using adversative and causal conjunctions. Besides, omission and misformation are the most common types of errors in the learners' use of conjunction. Both interference from first language and intra-lingual factors were found to be the sources of the learners' errors. Finally, several implications for teaching and learning conjunctions in Vietnamese high school context were drawn.