Abstract: Thematic organization plays a fundamental role in the message function of language. An important question is how translators deal with these thematic choices as textual devices when a text is translated into another language. Few studies have unfortunately brought their significance into focus. This study utilized primarily qualitative methods of data collection and analysis to examine the existence of thematic differences in English popular psychology texts and their translations in Persian. The researcher selected four hundred words based on Sical system from the first chapters of ten popular psychology books and investigated thematic development and progression in theme and their Persian translations.Applying Halliday’sthematic organization, the study revealed significant differences and similarities in the original texts and their translations regarding theme types. The study concluded thatthe results of this study can particularly inform translators in their decision-makings while translating in terms of selecting appropriate theme type, conveying the message more clearly as intended by the author, developing cohesion in discourse, and creating a cohesive text.