1. To investigate the errors of language structure and meaning that occurred bothin English-to-Thai and Thai-to-English translation.2. To find out the types of errors in English-to-Thai and Thai-to-Englishtranslation made by Mattayomsuksa 6 students.3. To compare the frequencies of errors between students with high and lowEnglish proficiency.4. To provide plausible explanations for the occurrence of the errors.