Introduction: Studies have shown that anxiety caused by the COVID-19 epidemic affects the patient’s decision-making processes regarding treatment and may even force him to refuse to continue treatment or disrupt the function and efficiency of treatment. This study aimed to determine the anxiety caused by COVID-19 and related factors in cancer patients undergoing radiotherapy. Methods: This analytical cross-sectional study included 125 cancer patients which sampled through convenience sampling from patients who admitted to radiotherapy centre in Yazd, Iran, in 2021. Patients had different primary cancers such as Breast, Lung, Head and Neck, Prostate, Stomach, Cervix, Blood, Intestine, Brain, Skin, Lymphoma, Kidney and Oesophagus. The COVID-19 Disease Anxiety Scale (CDSA) questionnaire was used to collect data. The frequency, percentage, mean (standard deviation), median (interquartile range=IQR) were used to description and the Mann–Whitney U and Kruskal-Wallis tests were used for data analysis. All analyses were performed in SPSS software (version 24) with a significance level of 5%. Results: In total, 55.2% of patients (69 patients) were male, 85.6% (107 patients) were married, and the mean (SD) age of participants was 50.16 (13.93) years. Median (IQR) the scores of mental, physical and anxiety symptoms were 11 (13), 4 (8), and 16 (18), respectively, which were moderate in physical and psychological symptoms according to guidelines. In terms of total anxiety classification, total anxiety score in our study was in mild category. Median of the CDSA score was significantly different in levels of age, education, economic status, and type of job (p<0.001). Conclusion: COVID-19 anxiety in cancer patients was in the mild level. Young people and people with lower social, economic, and educational status are more anxious. However, in addition to targeted safety and prevention measures, training and counselling programs may also be useful in reducing anxiety level.