The research aimed to investigate the core competency elements and the factors affecting the performance efficiency of the civil service teachers in the northeastern region, Thailand. The research procedure consisted of two steps. In the first step, the data were collected using a questionnaire with the reliability (Cronbach's Alpha) of .90. The sample size was specified based on Krejcie and Morgan’s table and 352 samples were selected using the simple random sampling technique. The second step involved an in-depth interview of 10 specialists to gain the ways to develop effective performance competency and the statistics used for data analysis included percentage, standard deviation, and descriptive analysis. The questionnaire results revealed that there were 11 elements of 4 core competencies. There were 2 elements for the achievement motivation competency, 3 elements for the service mind competency, 3 elements for the team work competency, and 3 elements for the self-development competency. For appropriate ways to develop core competencies for effective performance of civil service teachers, there were 13 ways which included understanding of related environment, supporting of working resources, having effective management system, reducing losses at work, creating good working atmosphere, promoting participation in decision making, supporting staff for professional development, communicating openly within the organizations, listening to the opinions of others, developing of quality system in work, creating good human relations among staff, building morale in the workplace, and specifying appropriate working patterns.