This study investigated principals’ openness and agreeableness as correlate of their administrative effectiveness in secondary schools in Anambra State. Two research questions guided the study and two hypotheses were tested at 0.05 level of significance. A correlational survey research design was adopted for the study. The population of the study consisted all the 263 principals in the six education zones of Anambra State. All the principals were used for the study. Two sets of researchers’-developed instrument titled “Principals’ Openness and Agreeableness Questionnaire” (POAQ) and “Principals’ Administrative Effectiveness Questionnaire” (PAEQ) were used for data collection. The instruments were validated by three experts and subjected to internal consistency test using Cronbach’s Alpha method which yielded 0.81 for POAQ and 0.89 for PAEQ. Pearson’ Product Moment Correlation Co-efficient was used to analyze data for the study. The findings of the study indicated that there is a strong positive relationship between principals’ openness and agreeableness and their administrative effectiveness in secondary schools in Anambra State. Based on the findings it was recommended among others that there is need for Post Primary School Service Commission to determine the personality traits of would be principals before their appointment to ensure they possess requisite traits to manage the affairs of the schools for effectiveness