This paper discusses the teaching quality of English teachers in Islamic high schools (Madrasah Aliyah) in Indonesia. The paper focuses on teachers' performance and professionalism. Empirical studies suggest that teachers' teaching quality significantly influences students learning achievement (Darling-Hammond, 2000; Liu, 2013; Newsome, 2016). It is debated that the teaching quality of English teachers at Madrasah Aliyah in Indonesia is lower than teacher quality in other mainstream (secular) schools. The observation is supported by the statistical data of students' achievement scores between Madrasah Aliyah and secular high schools in the English test as part of the National Examination. It is found that the outcomes are lower for the Madrasah schools (MoNE, 2016). The trend remains similar since 2014-2016. There are at least two possible factors that explain the outcomes and the link to English teachers' quality in Madrasah Aliyah: teachers' English mastery and their pedagogical knowledge. Furthermore, the the fact that there is less control and evaluation from school management, government, and other stakeholders would exacerbate the situation. Therefore, the paper intends to address two research questions: How do teachers of Madrasah Aliyah implement their English mastery? and How effectively do they teach English in the classroom? This is a qualitative study and observations and semi-structured interviews were conducted to collect the data. Classroom and school observation derives the information about teachers' quality in teaching English. Further, the school observation details the environment of madrasah including the activities of teachers, school facilities, and school management. The semi-structured interviews examine perspectives and recommendations about the school elements including students, teachers, and the head master regarding teachers' quality in Madrasah Aliyah. This paper is part of an ongoing research project on the quality of Madrasah Aliyah in Indonesia. The study will provide insights on improving the quality of English teachers in Madrasah Aliyah and provide recommendations to policy makers and stakeholders in Indonesia.