This study examined the effect size of the factors related to expertise through meta-analysis, focusing on previous studies related to the expertise of preschool teachers. The results of this study are as follows. First, it was found that the overall effect size of the factor group affecting the expertise of preschool teachers has a large effect size. Second, among the factors affecting the expertise of preschool teachers, job characteristics-protection were found to have the greatest effect size, followed by organizational characteristics, personal characteristics, job characteristics-risk. Third, it was found that among the individual characteristics of the professionalism of preschool teachers, personality and happiness showed a large effect size. Fourth, as for the effect size of early childhood teachers professionalism and organizational characteristics, participation in decision-making was found to have the greatest effect, followed by organizational effectiveness, peer relations, emotional leadership, director transformational leadership, and director transactional leadership. Fifth, the effectiveness of preschool teachers professionalism and job characteristics–protection factors were found to have a large effect on teacher efficacy, faculty commitment, teaching profession, play teaching efficacy, role performance, interaction with infants and toddlers, and job satisfaction. The belief was found to have a medium effect size. Finally, it was found that the job characteristics of preschool teachers professionalism-job stress, which is a group of risk factors, showed a medium effect size. In conclusion, this study is meaningful in that it presented objective results by organizing the results of previous studies related to the expertise of preschool teachers within one frame. In addition, it is meaningful that it provided basic data for detailed measures and follow-up research for the improvement of the professionalism of early childhood teachers