The objective of this research was to study of  leader behavior affecting on school effectiveness of private pre-elementary schools in Bangkok under Bangkok Archdiocese. The population of the study were 369 headmasters and teachers. Data collection was the questionnaires. Statistics for data analyzing included percentage, mean, standard deviation, and multiple regression. 
	The study revealed that 1) overall leader behavior was in high level .When considered in each aspects, it was found that relationship-oriented behavior had the highest mean in working happiness of teacher aspect. 2) Task-oriented behavior was found at high level in work improvement for effectiveness. The analyzed result about effectiveness of private schools showed that when considered overall, there was at high level. When ranked out from highest level to the least level of mean, there were school adaptation abilities, teacher unities, and learning achievements. 3) It had also found that leader behaviors were related to the effectiveness of private schools in pre-fundamental level in the archdiocese of Bangkok with significance at .05 level
