The present study aimed at analyzing writing errors caused by the interference of the Persian language, regarded as the first language (L1), in three writing genres, namely narration, description, and comparison/contrast by Iranian EFL students. 65 English paragraphs written by the participants, who were at the intermediate level based on their performance on a quick placement test, were analyzed by using Error Analysis (EA). The ideas of 15 teachers with more than six years of teaching in the field of TEFL were also sought through interview to validate the collected data. The results revealed that the first language interference errors fell into 12 categories: "singular/plural form," "modal auxiliary," "subject-verb agreement," "verb tense," "infinitive gerund," "pronoun," "article," "verb form," "prepositions," "sentence structure," "word choice," "comparison structure," respectively, and the number of frequent errors made in each type of written tasks was apparently different with regard to text genres. In narration, the five most frequent errors found were singular/plural form, modal auxiliary, subject-verb agreement, verb tense, and infinitive gerund, respectively, while the five most frequent errors in description were article, subject-verb agreement, modal/auxiliary, verb tense, and prepositions. The category for that of comparison/contrast comprised verb tense, singular/plural form, preposition, and subject-verb agreement respectively. The three errors of singular/plural form, verb tense, subject-verb agreement were the most frequent errors. The results reveals that different structural features required in a genre influences the writing errors made in the genre. Moreover, some pedagogical implications are provided based on the results.