Objective To investigate the changes on apparent diffusion coefficient(ADC)of diffusion weighted imaging with dif-ferent b values and to assess the diagnostic value of ADC for discriminating malignant from benign orbital masses.Methods 81 pa-tients (55 benign orbital masses and 26 malignant orbital masses)were enrolled.These patients underwent serial MR and echo-pla-nar DW imaging examinations of the orbits with b values=0 and/or 400,700,1 000 s/mm2 .The differences of ADC values under different b values between benign and malignant orbital masses were observed and compared,and diagnosis of ADC values was as-sessed by the receiver operating characteristic (ROC)curve.Results In three groups,the ADC values of benign orbital masses [(1.56±0.35)×10 -3 mm2/s,(1.40±0.34)×10 -3 mm2/s and (1.30±0.32)×10 -3 mm2/s,respectively]were significantly differ-ent (P <0.01),while the ADC values of malignant orbital masses were not statistically different (P =0.093)[(1.12 ±0.47)× 10 -3 mm2/s,(0.92±0.42)×10 -3 mm2/s and (0.87±0.40)× 10 -3 mm2/s,respectively].The areas under the ROC curve of ADC values were 0.799,0.866 and 0.867,respectively.The threshold values were 1.05 × 10 -3 mm2/s,0.94 × 10 -3 mm2/s and 0.93 × 10 -3 mm2/s (while sensitivity were 65%,81% and 81%;specificity were 94.5%,91% and 85%;positive predictive values were 85%,80%,72%,negative predictive values were 85%,91%,90%,and accuracy were 85%,87% and 84%,respectively).Conclusion The b value of 700 s/mm2 offered better diagnostic performance.ADC values of diffusion weighted imaging can reflect diffusion characteristics of the orbital masses,as a complementary tool in the differentiation of malignant from benign orbital masses.