Purpose: This descriptive correlation study was to identify the influencing factors on fatigue of patients undergoing radiotherapy for breast cancer. Method: One hundred twenty-six breast cancer patients were recruited from a radiotherapy clinic of two university hospitals. Data were collected using a package of questionnaires consisting of the Symptom Experience Scale, mood disturbance scale, and Social Support Scale. Results: The mean score of the fatigue, physical status, emotional status, and social support was 4.18, 21.96, 100.95, and 3.70, respectively. Family support and health profession support were 3.94 and 3.47, respectively. Fatigue and physical status, fatigue and emotional status, physical status and emotional status were found to have statistically positive correlations. But emotional status and social support were found to have statistically negative correlation. Stepwise multiple regression analysis showed that the influencing factors on fatigue were emotional status, marital status, and physical status which explained 64.4% fatigue of breast cancer patients.