The study aims to develop and psychometric validate Patient-Reported Outcomes Measures for Assessing Comfort during Chemotherapy in Breast Cancer Patients (PROMs BCC-20). This study was conducted in two phases: 1) items were developed from the literature review and in-depth interviews, and 2) Exploratory factor analysis (EFA), Confirmatory factor analysis (CFA), and concurrent validity were performed to evaluate construct validity. The participants were cancer stage I-IIIC, adult females, performance status was assessed by Eastern Cooperative Oncology Group (ECOG) score ≤ 2 after receiving the second cycle of adjuvant chemotherapy and selected by purposive sampling method. For each group of EFA and CFA was 250 participants. Five hundred breast cancer patients during adjuvant chemotherapy were recruited from three tertiary cancer centers. A succession of EFA using principal axis factoring with Promax rotation revealed four dimensions yielded a seven factors solution, explaining a 60.07 percent variance. CFA contains 20 items with five factors; 1) social function, four items; 2) digestive function, three items; 3) emotional function, six items; 4) environmental function, three items; and 5) sleep quality, four items via maximum likelihood with bootstrapping indicated a good fit model (SRMR = 0.045, RMSEA = 0.040, CFI = 0.947, and TLI = 0.935). The Cronbach's alpha of 0.86 demonstrated strong internal consistency reliability. Pearson's correlation coefficient showed acceptable criterion validity. The PROMS BCC-20 provides good psychometric properties and practical patients' direct reports of comfort in breast cancer patients during chemotherapy. The PROMs BCC-20 should be standardized for comfort measurement and tailor-made nursing care to provide patient satisfaction and good nursing outcomes.