The controversy surrounding the academic provision regarding the positive or negative effects of single-gender vs. coed classes has increased much more than ever. Because of sampling differences, data analysis differences and contextual differences, studies of single-sex versus coeducational classrooms has not yielded a clear set of conclusions. Having taught English for several years the researcher have clearly noticed that there is a significant difference between males, females and coed classrooms in terms of technical and vocational students' end-of-semester marks. Therefore, the present book endeavors to shed light, as much as it can, on the effect of single-sex/coed class composition on the English learning of Iranian technical and vocational students.The statistical analysis of the results shows that single-sex classes appear to be most advantageous for females whereas males performed better in coed classes. Males' class showed the weakest performance.