The purpose of this mixed methods research was to investigate the transformational leadership role of principals in Ethiopian secondary schools. The research examined the existence of transformational school leadership behaviours, strategies that leaders were employing and the relationship between transformational leadership roles of the principal and student achievement (10th grade GESLCE). Mixed methods involving qualitative and quantitative methods of data collection were used in order to triangulate the data and increase the validity of empirical material. The student achievement was assessed on the basis of on the percentage of passes in standardized assessments for the school year (2012/2005-2015/2007). Secondary schools in South Nations Nationalities and Peoples’ Regional State (SNNPRS) whose principals had been the incumbent for three or more years were selected purposively. A total of 80 surveys were distributed to school principals and 680 survey questionnaires to teachers in identified schools. Seventy-six school principals and 642 teachers returned the surveys representing 95% and 94.4 % return rate for principals and teachers, respectively. Interviews and focus group discussions were conducted with supervisors and teachers who worked with principals. Participants were informed of the objectives of the study and assured of the confidentiality of the research both in formal letters and in person. Factor analysis, means, standard deviations, Pearson and Spearman rho correlations, t-test and standard multiple regression were used to analyse quantitative data. In contrast, qualitative data were analyzed by grouping together common themes in each category and constructing a description of the participants’ views and perspectives. The findings revealed that transformational school leadership and its dimensions exist in secondary schools of SNNPR, Ethiopia. Among the six dimensions (namely; sharing school vision and building consensus, high performance expectation, individualized support, intellectual stimulation, modelling behaviour and building collaborative structure and strengthening school culture) three of them were ranked highly. These are, building collaborative structure, strengthening school culture and sharing school vision. On the other hand, building consensus and modelling behaviour had moderate score whereas, individualized support had relatively very low rank. Transformational leadership was accomplished through involving staff to be part of the mission, encouraging staff to come up with new initiatives, challenging the staff to meet beyond agreed standards, building trust, coach and support at individual level, sharing tasks as well as power, acting ethically and celebrating achievements to make their school effective. The relationship between transformational school leadership and student achievement showed that there is positive and moderate relationship, although it was statistically not significant. Socio-demographic data such as number of students in a school showed a negative correlation with student achievement. It is concluded that there was relationship between the principals’ transformational leadership role and student achievement in secondary school of SNNPRS of Ethiopia. Finally, the research calls for the principals to focus on more moral and material assistance to their staff’s professional growth, significant attention to the preparation and development of principals and adoption of transformational school leadership model in secondary school. The findings of the study add to limited but growing body of research on student achievement and the role of the transformational school leadership of the principal in school effectiveness. / Educational Leadership and Management / D. Litt. et Phil. (Education Management)