Pienemann (1998, 2015) put forward Processability Theory to enlighten why language learners follow definite developmental paths. The aim of the present study was to run a comparative investigation into the difficulty order of different grammatical structures for male and female Iranian EFL learners predicted by Processability Theory. 185 Iranian university students took part in this study. They received a Demographic Questionnaire and a Validated Researcher-Made Grammar Test designed based on the stages of Processability Theory. Item Response Theory (IRT) Rasch Modeling was used to analyze the collected data. Results pertained to the research questions revealed that the stages predicted by Processability Theory do not account for the Iranian male/female EFL learners. Another major finding emerged from the data was that the difficulty level of different grammatical structures presented by Pienneman in PT doesn’t match the difficulty order obtained in this study by male/female EFL respondents. All things considered, results of the study provided a reliable counterevidence for the assumptions of the theory.
