The purpose of this study is to investigate the relationships between junior high school teachers' personality traits and teaching effectiveness, and to understand the predictability of teaching effectiveness by personality traits. The researchers issued self-compiled personality traits scales and teaching effectiveness scales to 447 junior high school teachers around Taiwan who volunteered to be the subjects. Data was analyzed by canonical correlation analysis, Pearson's product-moment correlation analysis, and step-wise multiple regression. The major findings are as follows:1. Significant correlations were found between junior high school teachers' personality traits and teaching effectiveness. Conscientiousness along with Agreeableness in personality indicated higher correlation with factors of Material Presentation, Teaching Fluency, and Class Climate.2. 46.5% of the variation in junior high school teachers' teaching effectiveness can be predicted by factors of Conscientiousness, Agreeableness, and Openness to Experience in personality.Suggestions are provided for school teachers, school administrations, and future studies.