Purpose: To test the validity and reliability of The European Organization for Research and Treatment ofCancer (EORTC) core (QLQ-C30) and breast cancer module (QLQ-BR23) for Turkish breast cancer patients.Patients and Methods: A total of 127 patients treated with radiotherapy (RT) enrolled to this prospective study.EORTC QLQ-C30 and QLQ-BR23 modules applied to patients before initiation of RT and at follow-up period.Statistical analyses were performed by SPSS 13.0. Results: Questionnaires’ were found reliable and valid forTurkish breast cancer patients. Six of the 8 multi-item scales of QLQ-C30 had a high reliability (Cronbach’s α>0.7); where physical functioning and pain scores were less reliable (Cronbach’s α of 0.66 and 0.68 respectively).In the QLQ-BR23, 3 of 5 multi-item scales were reliable; less reliable were breast and arm symptoms scale(Cronbach’s α of 0.65 and 0.61 respectively). In our analysis the most determinative subscales of QLQ-C30on global health was emotional functioning followed by fatigue, role functioning and appetite loss (respectivelyp=0.002, p=0.01; p=0.03 and p=0.08). Among QLQ-BR23 scales systemic therapy side effects, future perspectiveand upset by hair loss subscales had high impact on global health status (respectively p=0.006; p=0.01 and p=0.03). Conclusions: The Turkish version of EORTC QLQ-C30 and QLQ-BR23 modules are reliable and valid tools toassess quality of life of Turkish breast cancer patients.