This study aimed at analyzing writing errors caused by the interference of the Thai language, regarded as the first language (L1), in three writing genres, namely narration, description, and comparison/contrast. 120 English paragraphs written by 40 second year English major students were analyzed by using Error Analysis (EA).The results revealed that the first language interference errors fell into 16 categories: verb tense, word choice, sentence structure, article, preposition, modal/auxiliary, singular/plural form, fragment, verb form, pronoun, run-on sentence, infinitive/gerund, transition, subject-verb agreement, parallel structure, and comparison structure, respectively, and the number of frequent errors made in each type of written tasks was apparently different. In narration, the five most frequent errors found were verb tense, word choice, sentence structure, preposition, and modal/auxiliary, respectively, while the five most frequent errors in description and comparison/contrast were article, sentence structure, word choice, singular/plural form, and subject-verb agreement, respectively. Interestingly, in the narrative and descriptive paragraphs, comparison structure was found to be the least frequent error, whereas it became the 10th frequent error in comparison/contrast writing. It was apparent that a genre did affect writing errors as different text types required different structural features. It could be concluded that to enhance students’ grammatical and lexical accuracy, a second language (L2) writing teacher should take into consideration L1 interference categories in different genres.