The aim of this research is to empirically investigate the relationship between a leader’s self-efficacy, transformational leadership and leader effectiveness. On the basis of the literature and current leadership research review, the theoretical model, in which transformational leadership mediates the relationship between a leader’s self- efficacy and effectiveness, is proposed. The research was conducted within a three-month-long management simulation game at two Czech universities. A total of 32 leaders participated together with 604 subordinates. The criteria of leader effectiveness included leadership emergence and perceived leader effectiveness, both assessed by the leaders’ subordinates, and objectively measured by group performance. For the assessment of the transformational leadership approach of leaders, we administered a questionnaire based on the transformational leadership theory to their subordinates. The self-efficacy of leaders was measured by a self-report questionnaire filled by leaders. The relationship of self- efficacy to transformational leadership was not supported, as well as the association between leader’s self-efficacy and criteria of leader effectiveness. Therefore, the mediation model with transformational leadership in the role of the mediator was not accepted.