The study aimed to identify the degree of instructional leadership practice among principals of post-basic education schools and its relationship to the professional performance of teachers in the Sultanate of Oman. To achieve the objective of this study, a qualitative approach using surveys was utilized to collect data after verifying its validity and reliability. The sample of the study consisted of 344 male and female teachers of post-basic education schools (grades 11-12) in the Governorates of Muscat and Al Dakhiliyah, in the Sultanate of Oman. The study reached a set of results, most notably. Based on the teachers' perspectives, there is a significant relationship between the instructional leadership practices of post-basic school principals in the Sultanate of Oman. The study also found that there were no statistically significant differences at the significance level (α ‏‎ ‏≥‏0.05) In the responses of the study sample about the degree of instructional leadership practice among principals of post-basic education schools based on the study's control variables (governorate, educational qualification, and years of experience). In addition, the findings showed statistically significant differences at the significance level (α ‏‎‏≥‏ ‎0.05) in the degree of instructional leadership practice that is attributed to the gender variable in favor of females. The study also concluded that the degree of effectiveness of the professional performance of teachers is high. Results of the study also showed that there were no statistically significant differences at the significance level (α ‏‎‏≥‏ ‎‏0.05) In the responses of the study sample about the degree of effectiveness of the professional performance of teachers in post-primary education schools based on the study's control variables (governorate, academic qualification, and years of experience). However, there were statistically significant differences at the significance level (α ‏‎‏≥‏‎ 0.05) in the degree of the professional performance effectiveness based on the control variable gender in favor of females. There also was a positive, medium, and statistically significant correlation between the degree of school principals' practice of instructional leadership, and the degree of effectiveness of teachers' professional performance. In light of the findings of the study, it is recommended to expand the training of school principals on instructional leadership due to the positive relationship between instructional leadership training and the professional performance of teachers. It is also recommended that a systematic program of exchange visits between male and female schools is established in order to exchange experiences between teachers in the two dimensions of professional performance.