Abstract： Objective To investigate the diagnostic value of ADC for breast lesions with different enhancement shape or mass size. Methods One hundred and thirty-six breast lesions confirmed by histopathology were included in this study. According to enhancement shape and size of the lesion, all lesions were divided into 3 groups: non-masslike enhancement ( G1 ), masslike enhancement with the largest diameter ＜ 2. 0 cm (G2a) and masslike enhancement with the largest diameter ＞ 2. 0 cm (G2b). Echo planar imaging DWI was performed and three b-values (0,500 and 1000 s/mm2) were applied. The t-test was used for testing the difference of ADC between malignant and non-malignant breast lesions in each group. ROC curve was deduced to test the diagnostic efficiency of ADC. The sensitivity, specificity, negative predictive value( NPV), positive predictive value(PPV) and accuracy of ADC for the diagnosis of breast lesions were calculated under the different threshold. Appropriate b value and threshold were determined with the combination of morphologic evaluation. Results There were no significant differences for the mean ADC values between malignant [b =800 mm2/s: ADC value = ( 1.13 ±0. 23) × 10-3 mm2/s,b=1000 mm2/s: ADC value = (1.05 ±0.20) × 10-3 mm2/s] and non-malignant breast lesions [b =800 mm2/s: ADC value = ( 1.28 ±0. 27) × 10-3 mm2/s, t = 1. 636, P =0. 112,b = 1000 mm2/s: ADC value=(1.20 ±0.23) × 10-3 mm2/s, t = 1.720, P =0. 109] in Group 1. The mean ADC values of malignant breast lesion [b =800 mm2/s: ADC value = (1.07 ±0. 15) × 10-3 mm2/s,b = 1000 mm2/s:ADC value = (0. 99 ±0. 14) × 10-3 mm2/s] were significantly lower than that of non-malignant lesion [b =800 mm2/s: ADC value = ( 1.37 ± 0. 37 ) × 10-3 mm2/s, t = 4. 803, P = 0. 000; b = 1000 mm2/s: ADC value= (1.30 ±0.34) × 10-3 mm2/s, t =4.227, P =0.000] in Group 2a. The mean ADC values of malignant breast lesion [b =800 mm2/s: ADC value = (0. 97 ±0. 14) × 10-3 mm2/s; b = 1000 mm2/s:ADC value = (0. 93 ±0. 14) × 10-3 mm2/s] were significantly lower than that of non-malignant lesion [b =800 mm2/s: ADC value = ( 1.40 ± 0. 39) × 10 -3 mm2/s, t = 4. 227, P = 0. 000; b = 1000 mm2/s: ADC value = ( 1.35 ±0. 36) × 10-3 mm2/s, t =4. 329, P =0. 000] in Group 2b. The diagnostic efficiency was equal( x2 =0. 36,P =0. 5460) whenever b value of 800 or 1000 s/mm2 was selected. The highest sensitivity (97.7%) and NPV (97. 1%) were obtained with b value of 1000 s/mm2 and threshold of 1.25 ×10 -3 s/mm2. Conclusion MR DWI is useful for the differential diagnosis of breast lesions with masslike enhancement rather than nonmasslike enhancement.