Objective To compare the accuracy of full field digital mammography (FFDM) and digital breast tomosynthesis (DBT) in predicting size of ductal carcinoma in situ (DCIS).Methods Totally 30 women pateints with DCIS confirmed by histology were analyzed retrospectively.Two radiologists measured the size of the lesions on FFDM,DBT images respectively,and compared with pathological results.Two kinds of inspection methods correlation with pathology were compared with Pearson correlation analysis.Results Size of DCIS measured by doctor 1 and doctor 2 between FFDM and histology had correlation (r=0.857,0.818,both P＜0.01),and those between DBT and histology also had correlation (r=0.924,0.885,both P＜0.01).Intraclass correlation coefficient (ICC) of two doctors was 0.947 on FFDM,and 0.908 on DBT.Conclusion DBT provides a better estimation of the size of DCIS.Two physicians had high consistency for lesion size evaluation.For DCIS with microcalcifications,both FFDM and DBT provide smaller estimations compared with pathology,thus a bigger surgical resection of lesion is needed.