Abstract This study investigated the ESP teachers' in-service training needs at the Omani Armed Forces. This is a quantitative study. There are 4 independent variables which are teachers' nature of duty (uniformed teachers vs civilian teachers), Qualification (BA, MA, CELTA and others), position (teachers vs supervisors) and years of experience. The dependent variables in the study are the teachers and supervisors responses. The sample consisted of 101 teachers and 28 supervisors from the Military Educational Institutes in the Armed Forces. Both teachers and supervisors answered a five Likert scale questionnaire and 7 teachers participated in an interview. The study aimed at answering the following questions: 1- What are the in-service training needs of Omani ESP teachers from the perspective of teachers and supervisors? 2- Are there any significant differences among teachers in the types of ESP in-service training needs due to: the nature of duty (uniformed vs civilian), qualifications, position (teachers vs supervisors) and years of experience? The statistical analysis of the data and the focus interview revealed: 1. Generally, there is an equal need of in-service training from the perception of both teachers and supervisors. 2. ESP vocabulary, content-specific knowledge, developing oral skills, teaching methods/techniques, testing and evaluation and need analysis are areas placed at the critical need level. 3. Knowledge/skills on how to design and use questionnaires, interviews and surveys and Knowledge/skills about how to design curricula for ESP courses are areas with less training needs. 4. With reference to the variable nature of duty it was found that there was a significance difference between means of uniformed teachers and civilian teachers in favor of civilian teachers with a mean greater than uniformed. 5. There are no significant differences in terms of the training required for MA, BA, CELTA or other qualification holders. 6. Regardless of their positions supervisors and teachers assumed very similar level of training needs. 7. With regard to years of experience it was found that there was no significant difference.