Research in the United States show that professional learning (PL) can improve classroom instruction. However, teachers, especially educators in the content areas, receive insufficient training specific to meeting the language and academic needs of English learners (ELs). The purpose of the current randomized controlled trial (RCT) study was to introduce a high-quality PL program for supporting middle school teachers in teaching science to ELs through classroom observations, so as to evaluate whether in-service science teachers' instructional delivery can be shaped by intensive, structured, and ongoing PL. Results indicated that after receiving PL, treatment teachers allocated a significantly higher portion of their instructional time in teaching content language, developing students' expressive language skills, small group and pair activities, implementing strategies evidenced to effective with ELs, and providing more opportunities for students to participate in hands-on activities that were content-related and grade-appropriate. Practical implications for PL were discussed for an international audience.