The number of English language learners (ELLs) is increasing rapidly at US schools. Mainstream teachers have an important role in academic success of these students because ELLs spend most of their school time in mainstream classes, and pulled-out a few hours in a school day for ELL services. However, research on mainstream teachers and ELL education highlights that mainstream teachers may have negative perceptions of ELLs, and they may use wrong accommodations to teach these students due to their lack of knowledge in ELL education. This study aims to understand how taking two online classes on ELL education changes mainstream teachers' perceptions of this student group. The results showed that mainstream teachers changed their perceptions of ELLs greatly when they took these two online classes, and online discussions had a great role in this perception change. Some implications to develop online ELL education courses for pre-service and in-service teachers are also provided.