This study aimed at classifying the grammar errors found in business writing assignments produced by Thai EFL students who took an English Correspondence course at the Language Institute, Thammasat University, Thailand. Eight business documents, which included memoranda, letters, and e-mails, written by each of 26 Graduate Diploma students were analyzed by using the Error Analysis (EA) Model by Carl James (1998). The results revealed that the most-frequently found grammar errors fell into ten categories: (1) subject-verb agreement, (2) sentence fragments, (3) noun plurals, (4) noun singular, (5) tenses, (6) spelling errors, (7) active-passive voice, (8) misselection of the article and comma splices, (9) infinitive and noun morphology errors, and (10) gerunds. It was also found that the type of documents (i.e. memos, e-mails, and letters) had no effect on the number of grammar errors made by the student, since they didn’t have problems with document formats. The frequency of grammar errors appeared was quite consistent in all of these document types. It could be inferred that these grammar errors were as a result of the students’ overgeneralization. They need to improve their lexical and syntactical skills. To enable them to succeed in writing business messages, teachers should provide them with ongoing input that comes in a variety of forms: reading input, listening and speaking input, writing input as well as appropriate corrective feedback on their grammar errors. In this way, teachers may be able to help EFL students make progress in their writing skills and produce fewer grammar errors.