Currently, there is a huge demand for secondary school education worldwide especially in the developing countries such as Vietnam. School leadership is occupying a central position in educational reform. In order to meet this for the secondary school education, the Vietnamese government tries to develop effective school leadership in its education policy agenda. The aim of this study is to investigate the influence of factors on instructional leadership of principals in lower secondary education. The study was completed with 97 secondary public schools teachers and principals in the North of Vietnam. A quantitative research design was used to conduct this study. The data was analyzed through factors such as internal and external from the role of principal in instructional leadership. Research findings underline that instructional leadership is had directly by external such as "The degree of autonomy of the school for instructional activities", "Awareness of the managers and teachers about leading teaching activities" and "education innovation" more internal such as "System of guiding documents of management levels on teaching activities"; "Conditions of facilities and finance for school teaching activities" and "Principal's training level" Principals should recognize the change of the context and enhance the improvement of teachers' shared responsibility and partnership in school decision making. Therefore, the Vietnamese government should make sure to have many positive issues that may have effective instructional leadership capacity in order to improve the culture of teaching and learning in Vietnames secondary schools.