Abstarct: Abstract The inconsistency in the learners’ compositions is partly rooted in deictic errors that they make in their compositions. To date, no previous study has uncovered the reasons for these errors. Therefore, 80 compositions were analyzed to identify and categorize different kinds of deictic errors. The causes of errors were also predicted. To do this, the present study followed a descxriptive-analytic approach. The data were analyzed using error analysis and contrastive analysis methods. The pragmatic theories of deixis were considered as the theoretical frxamework of this study. Analyzing the data, 186 deictic errors were identified. These errors were categorized as follows: Person deictic errors (67 errors), temporal deictic errors (45 errors), place deictic errors (38 errors), and discourse deictic errors (36 errors). It was also found that deictic errors were made because of two key factors: intralingual factors and interlingual factors. Among intralingual factors, time deictic errors and discourse deictic errors were the most and the least frequent error types, respectively. The intralingual factors were as follows: (i) overgeneralization of deictic rules; (ii) simplification of deictic structures; (iii) ignorance of English deictic rules. Through the influence of Persian deictic patterns, person deictic errors happened the most, and time deictic errors, the least. Errors made due to negative transfer of Persian patterns were as follows: (a) person deictic exxpressions (omitting direct object pronouns after the verb such as to help, to tell, and to deal with, replacing direct object pronouns with indirect object pronouns, lack of distinguishing the first, second, and third types of person pronouns from each other, (b) time deictic exxpressions (using Persian structure like to now in English, using Persian structure such as the next day after the day I’ve told in English, and using three tense markers in a sentence), (c) place deictic exxpressions (using singular demonstrative pronouns with plural nouns and conversely), (d) discourse deictic exxpressions (using a rule such as that insertion rule to connect two sentences, using the mentioned rule in the superlative adjective clause, using a time exxpression together with a discourse exxpression to refer to the referent, and substituting a demonstrative such as that for prepositions). The uncovered causes of deictic errors provide valuable insights for stakeholders who care about the quality of students’ writing in Iran.