Radiotherapy is a frequently prescribed and highly effective form of treatment of oncology patients. However, many patients feel rational or irrational fear of the application of radiotherapy, which may provoke mental and physical stress, anxiety, growing anger and hostility, thus reducing quality of life. The aim of this study was to develop, reliability test and validate a questionnaire for assessing the level of fear of radiotherapy in oncology patients. We performed a prospective qualitative study based on the development, validation and reliability testing of the questionnaire developed for assessing radiotherapy-caused fear in oncology patients treated in the Centre for Oncology and Radiology, Department of Radiotherapy, Clinical Centre Kragujevac. The study included 154 patients and the final version of the questionnaire integrated 15 questions. After the elimination of inappropriate questions the Cronbach coefficient α was 0.946. The questionnaire consists of two factors which represent 57.423% and 6.925%, making a total of 64.348% of the variance of the questionnaire. The results of our study show that the questionnaire used is a unique, reliable and valid instrument for assessing the level of fear of radiotherapy in oncology patients the application of which will allow us to identify patients with elevated levels of fear of radiotherapy.