The purpose of the present study is to find out whether Iranian high school English textbooks are sexist textbooks which discriminate between male and female students based on their sex. Halliday's systemic-functional linguistic theory was used as an innovative framework to language gender bias studies. To this end, four English textbooks currently taught in the Iranian high schools were chosen as a sample of this study. The linguistics analysis revealed that there were more males in the Theme and Rheme position. In other word, males had dominated the communicative positions of the clauses in the textbooks. Also the outcome of the participant role analysis showed that the frequencies of males for different participant’s roles were more than that of females. Thirty males played the role of actors where as only twenty females were in the position of actor throughout the text of these textbooks. Eleven males were found in the role of senser in all clauses of the textbooks. The finding may be attributable to the writer `s attitude, culture, and traditional patriarchal system of Iranian society. Yet another reason why men are more visible than women may be related to the power of the influential policy makers (mostly men) who make curriculum decisions that favor boys and men.