Abstract Objectives To compare the ultrasound features and geometric roundness of triple negative breast cancer (TNBC) smaller than 2 cm and fibroadenoma9(FA), and to improve the diagnostic accuracy of TNBC smaller than 2 cm. Methods From January 2015 to December 2019, 74 patients underwent surgical treatment with pathologically confirmed TNBC smaller than 2 cm (TNBC group) and 125 FA (FA group) patients at the affiliated Wuxi People''s Hospital of Nanjing Medical University were included in this retrospective analysis. We evaluated the ultrasonotomograms from all patients and calculating the geometric roundness of the mass. The age, lesion size, shape, orientation, margin, echo pattern, posterior echo characteristics, calcification and other characteristics and geometric roundness of the two groups were compared. Results There was a significant difference in the average age between TNBC group and FA group(47.3 ± 10.7 vs. 35.5 ± 9.0, P<0.05). Compared with the FA group, the probabilities of irregular shape (66.2%, 49/74), non-parallel orientation (18.9%, 14/74), irregular margin (87.8%, 65/74), and rear echo enhancement (64.9 %, 48/74) were higher in TNBC group (P<0.05). Moreover, more masses in the TNBC group were classified as BI-RADS 4 and 5 categories, which was statistically different from the FA group (P <0.05). The average geometric roundness of the TNBC group (81% ± 9%) was significant greater than that of the FA group (66% ± 11%) (P <0.05). ROC curve analysis showed that the best cutoff value in differential diagnosis of these two groups was 80%, with the area under ROC curve (AUC) value of 0.816. The sensitivity and specificity were 77.2% and 74.6%, respectively. Multivariate analysis showed that advanced patient age, irregular shape, non-parallel orientation, posterior echo enhancement, BI-RADS classification into categories 4 and 5, and greater geometrical roundness of the lesion were independent influencing factors for TNBC smaller than 2 cm. Conclusions Compared with FA, advanced aged patients with TNBC smaller than 2 cm often show more suspicious ultrasound features and greater geometric roundness. These features may help to distinguish TNBC smaller than 2 cm and fibroadenoma.