Exploring the Effects of Class Size on Second Language Acquisition in Private Sector Pakistani Schools Abstract The current paper elucidates the ‘effect of class size’ on attitudes of teachers and students and its influences on the learning competences of not only a student, but imparting aptitudes of a teacher and the evolving role of both in general. The quantitative research is conducted through a detailed questionnaire filled by the teachers of private Pakistani schools to evaluate the effect of class size on students’ learning. Besides, the study examines the correlation between class size and its impact on students’ learning in the second language acquisition (SLA) classrooms. The findings of the study indicate that the larger classes contribute to the decline in teachers’ performance, which concurrently result to the decrease in students’ learning. Moreover, the findings of the current investigation in the Pakistani context are synonymous to those of the previous international researches, which signify that it’s the teacher, who is affected first in larger classrooms that ultimately effect students’ performance. The paper concludes that learners can therefore, benefit from smaller classes at middle level in terms of more individual attention and facilitating engagement in learning. Keywords: Class size, second language acquisition (SLA), teachers’ performance, students’ learning.