This research is aimed to study the direct, indirect and total influence of strategic leadership factors of school administrators influencing the effectiveness of small-sized schools. The samples were 1,396 administrators of small-sized schools under the primary educational service area office, the Basic Education Commission in the academic year 2017. The population was 15,999 small-sized school administrators who are administrating in small-sized schools under the primary educational service area office of the Basic Education Commission of Thailand. The research was conducted in the same academic year. It was found that Setting Organizational Direction (SETTD) had direct influence, an indirect influence, and a total influence on the effectiveness of most small-sized schools. In addition, strategy formulation (STRAT) had an indirect influence higher than the strategy implementation with the influence value of 0.044. The strategic control and evaluation (EVALU) had a direct influence and a total influence on the effectiveness of small-sized school with the influence value of 0.046. The model for developing the strategic leadership of school administrators of small-sized schools was obtained.