Purpose An effective biomarker for the diagnosis of breast cancer (BC) and benign breast diseases (BBD) is crucial for improving the prognosis. We investigated whether N6-methyladenosine (m 6 A) can be a diagnostic biomarker of BC. Materials and Methods We detected the contents of peripheral blood m 6 A in 62 patients with BC, 41 patients with BBD, and 41 normal controls (NCs) using the colorimetric method. The relative expression of the m 6 A regulated genes methyltransferase-like 14 ( METTL14 ) and fat mass and obesity-associated ( FTO ) was analyzed using quantitative real-time polymerase chain reaction. Results m 6 A in peripheral blood RNA was significantly higher in patients with BC than that in patients with BBD (p < 0.001) or the NCs (p < 0.001). m 6 A was closely associated with the disease stage (from stage 0 to stage I-IV, p=0.003). The receiver operating characteristic curve of m 6 A contained an area under the curve (AUC) value of 0.887 in BC, which was greater than that of carcinoembryonic antigen (CEA) or carbohydrate antigen 153 (CA153). The combination of m 6 A, CEA, and CA153 improved the AUC to 0.914. The upregulated and downregulated mRNA expression of METTL14 and FTO , respectively, might contribute to the increase of m 6 A in patients with BC. m 6 A combined with METTL14 and FTO improved the AUC to 0.929 with a specificity of 97.4% in the peripheral blood of patients with BC. Conclusion The peripheral blood RNA of m 6 A might be a valuable biomarker for the diagnosis of BC.