A breast cancer diagnostic composition and kit, and methods of diagnosing breast cancer or acquiring information for breast cancer diagnosis by using the composition or kit are provided. The composition or kit includes a polynucleotide that is the same as or complementary to at least one microRNA (miRNA) selected from the group consisting of hsa-miR-126, hsa-miR-23a, hsa-miR-24, hsa-miR-19b, hsa-miR-103, hsa-miR-142-3p, hsa-miR-144, hsa-miR-15a, hsa-miR-185, hsa-miR-93, and hsa-miR-30c in a vesicle, or a fragment of the microRNA.