The study aimed to identify the degree of school principals having a career maturity in the Governorate of Muscat and its relationships with a school's organizational effectiveness. This is though revealing the reality of the degree of school principals having a career maturity in the Governorate of Muscat and its relationships with a school's organizational effectiveness, and also though revealing the differences between the responses of sample members to demographic variables such as gender, academic qualification, years of experience, job title, and disclosure of the relationship between the career maturity of the school principals and the organizational effectiveness of the school. The descriptive approach has been used, where a questionnaire was prepared containing (60) clauses distributed in two parts. The first part concerns with career maturity in three areas: administrative, technical and social. The second part concerns about a school's organizational effectiveness. The questionnaire had applied to a sample of (450) between the head of the school, the assistant, and the first teacher of a number of schools in Muscat Governorate. Data was processed statistically using the SPSS statistical package. The study reached the following results: That the degree of school principals having a career maturity in the Governorate of Muscat came to a high degree in all dimensions of the scale. Where career maturity scale in the field of art ranked first, followed by career maturity scale in the field of administrative and finally career maturity scale in the field of social ranked last. The results showed that there were statistically significant differences at (α≤0.05) in the estimation of study sample on the degree of school principals having a career maturity in the Governorate of Muscat in Oman according to gender variable in all dimensions of the scale. Statistical differences came for the female sample in all dimensions, and for the holders of the bachelor's degree according to the variable of the academic qualification. The results also showed that there were no statistically significant differences at (α≤0.05) in the estimation of study sample on the degree of school principals having a career maturity in the Governorate of Muscat in Oman according to variables of years of experience and the job title is in all dimensions of scale. The results showed that schools in Muscat Governorate in Oman achieved a high level of organizational effectiveness with statistically significant differences at (α≤0.05) in the estimation of study sample on the degree of school principals having a career maturity in the Governorate of Muscat in Oman according to gender variable in all dimensions of the scale. Statistical differences came for the female sample in all dimensions, and for the holders of the bachelor's degree according to the variable of the academic qualification. The results also showed that there were no statistically significant differences at (α≤0.05) in the estimation of study sample on the degree of school principals having a career maturity in the Governorate of Muscat in Oman according to variables of years of experience and the job title is in all dimensions of scale. Moreover, the results showed that there is a statistically significant correlation relationship at the level of (α≤0.01) between the degree of school principals having a career maturity in the Governorate of Muscat and the level of organizational effectiveness of the school. This means the higher the level of the degree of school principals having a career maturity, the higher the level of their school's organizational effectiveness. The study came out with several recommendations, mostly important are to remain the enforcement of mechanisms, schematics and programs which aim to raise levels of career maturity of school headmasters especially to what the Specialized Center or Vocational Training offers to teachers as an effective role that resulted in the ascension of their performance and reflected positively on the effective organization of the school.As the career maturity of headmasters became high, the possibility of various goals will likely to be achieved.