The research investigates probable causes of translation errors by examining types of Thai-to-English and English-to-Thai translation errors and determining the most common translation errors. The participants of this study were 32 English for Communication students at a Thai University. The data used in this study was obtained from exercises and examinations of students. Data analysis was carried out using content analysis and sorted by frequency and percentage. The results obtained found that syntactic errors (65%) were the most frequent translation errors, followed by semantic errors (24.6%) and miscellaneous errors (10.4%), respectively. The causes of errors and translation problems were determined through interviews and stimulated recall. Translation procedures, low self-confidence, carelessness, and anxiety were the main causes of errors found. This study recommends that additional class time be provided to address all errors found. Moreover, group work and the implementation of authentic translation is proposed to increase self-confidence and decrease anxiety. It is hoped that this study results can improve translation teaching and the course "Translation from Thai into English" in Thailand.