INTRODUCTION: Several stress factors are experienced by emergency medical technicians (EMTs), the identification and management of which may be a major challenge due to the lack of valid and reliable instruments. This study aimed to develop a relevant and easy-to-use occupational stress scale (OSS) for EMTs with adequate psychometric properties. MATERIAL AND METHODS: A mixed method with an exploratory sequential design was used in this research. Items were generated based on the existing literature and a qualitative study, followed by testing the content and face validity of the items. Exploratory factor analysis (EFA) was done with a random sample of 247 EMTs. Also, internal consistency and stability reliability were investigated. RESULTS: From the initial 74 items, 20 with content validity ratio and content validity index were removed. In EFA, the item set resolved to a 50-item scale in the six dimensions include: Patient and family conditions, Environmental and occupational conditions, Traumatic consequences, Supportive management problems, Lack of support, and Interpersonal and individual tension. Cronbach's alpha and Intra-class correlation coefficient (ICC) showed excellent reliability. CONCLUSIONS: The OSS-EMT represents a psychometrically derived instrument that identified important stressors for EMTs., and is probably among the first studies in Iran. While explaining the methodology precisely, this study evaluated the validity and reliability of the OSS in EMTs based on principles of survey instrument development and validation
