The school administrators’ leadership is science and art in encouraging the teachers and educational personnel to work cooperatively until achieving the determined goal which will lead to a competitive advantage of the schools under the rapidly changing contexts of the society in a globalization era. The purposes of this research were to: 1) study the school administrators’ leadership level in the 21st century, 2) study the school effectiveness level, 3) study the relationship between the school administrators’ leadership in the 21st century and the school effectiveness, and 4) create a predictive equation of the school administrators’ leadership in the 21st century affecting the school effectiveness. The sample used in this research was a group of 327 teachers in the school under Chanthaburi Primary Educational Service Area Office 1. The sample size was calculate using Krejcie and Morgan’s comparison table and proportionate stratified random sampling method. The research instrument wes a five-level rating scale questionnaire with a reliability of 0.98. The statistics used for data analysis were: percentage, standard deviation, Pearson product-moment correlation coefficient and simple regression analysis.