This study aimed at investigating the effects of school principals' transformational leadership styles on teachers' self-efficacy. To achieve this aim, the researcher used a sample of 260 secondary teachers in Plaridel District, SDO-Bulacan during the academic year 2018-2019. The "Multifactor -- Leadership Questionnaire (MLQ) Form 5X" by Bass and Avolio and "Teacher's Self-Efficacy Scale 1 (long form)" by Megan Tschanmen-Moran and Mary Anita Woolfolk Hoy was used to describe the transformational leadership styles of school principals and the level of teachers' self-efficacy, respectively. The collected data were analyzed and treated statistically through the use of Statistical Packages for Social Sciences (SPSS). Results of the regression analysis indicate that all eleven variables of the transformational leadership styles of school principals affect teachers' self-efficacy to a different extent as shown by the non-zero coefficients. A closer look at the obtained B coefficients, one could deduce that two variables yielded B coefficients of 0.08 (effectiveness) and 0.06 (contingent reward) with associated probability less than the significance level set at 0.05. This means that effectiveness and contingent reward significantly affect teachers' self-efficacy that for every unit improvement in effectiveness and contingent reward, teachers' self-efficacy can be expected to increase by 0.08 and 0.06 respectively. The rest of the variables also affect the teachers' self-efficacy but not to a significant extent. The study recommended that school principals should continue to nurture their leadership skills rating and further school innovations and transformations were highly recommended.