This study was an attempt to investigate Iranian EFL learners' private speech across proficiency levels and gender while doing reading comprehension tasks. Moreover, it was an attempt to examine the different forms of private speech produced by Iranian English learners. Six forms of private speech were coded and analyzed: affective markers, sub-vocalization, switch from L2 to L1, repetition, unfinished sentences, and self-directed questions. It was a descriptive correlational research. The participants of this study were ninety female and male EFL learners at three levels of foreign language proficiency from two schools in Lahijan, Iran. They were selected from 110 students by administering an Oxford Quick Placement Test. Next, reading comprehension tests were administered to the participants. In order to gather data, each participant was equipped with a MP3 voice recorder to keep track of very low private speech markers while performing reading tasks. Pearson Chi-Square test was utilized to determine the probable relationship between the six forms of private speech and learners' proficiency levels and gender. The results revealed that there was statistically significant relationship between the forms of private speech produced by Iranian EFL learners and their foreign language proficiency. However, the outcomes did not yield a significant correlation regarding gender and the forms of private speech. It can be concluded that private speech is responsible for both regulating mental ability in complex tasks and facilitating internalization of mental ability. In second /foreign language learning, externalized private speech functions not just as a self-regulatory mechanism aiding problem-solving but as a tool in learning and internalizing the L2.