The study aimed at revealing the role of senior teachers in the process of evaluating teachers' professional performance in the interior province in Oman. The study was conducted using the descriptive approach in which a case study method is used. To achieve the objectives of the study the researcher used the interview as a tool for gathering data. The interview consisted of four main questions and it targeted a sample of twelve participants. They were schools principals, supervisors and senior teachers. The study's results, through its tools and research procedures, revealed that the role of senior teachers in evaluating other teachers' professional performance is very effective. In other words, the evaluation process is now more objective and credible, as the evaluation process is based on objective and scientific criterion. Therefore, each participant in the process is responsible in assessing the items and indicators that relates to the professional performance according to their specialties. As a result, the senior teacher is permanently and constantly knowledgeable of the teacher's situation, which helps to provide technical support because he or she can measure the pedagogical content knowledge more than anybody else can. At the end, the study recommended: 1- To design a special program to train and qualify senior teachers administratively because there are some challenges that senior teachers encounter as indicated in chapter four. 2- To design a comprehensive guide that clarifies the procedure used to assess the professional performance of teachers.