Writing an academic paragraph has been considered as a challenging task for EFL learner since it requires learners the capacity to utilize language correctly and communicate thoughts coherently. Although researches on language errors in English composition have been conducted, less attention has been paid to errors in paragraph unity. Applying Error Analysis, this study scrutinizes comprehensively and presents the types and frequencies of errors related to unity in 65 paragraphs produced by first-year English majored students at University of Foreign Languages and International Studies, Hue University. All the errors committed by the students were analysed using error analysis classification by Liu and Wang (2011). Analyses of these errors revealed that among the three factors breaking paragraph unity, irrelevant supporitng sentences accounted for the largest percentage (85,5%), followed by ineffective concluding sentence (8,5%) and errors in topic sentence (6,0%).