Objective To evaluate the value of diffusion weighted imaging in diagnosis of ovarian cystadenocarcinoma. Methods 28 patients with pathologically-proved cystadenocarcinama and 30 patients were cystadenoma based on MR imaging were retrospectively analyzed, record the DWI signal of the cystic wall or interval and measuring its ADC value (b=800s/mm2 ) and nonparametric test was used. Results The MR findings of cystadenocarcinoma were complex solid and cystic ovarian tumors with thick,irregular wall with thick sepa,mural nodule,and soft tissue component,which showed hyperintensity on DWI,and the mean ADC values was (0.95±0.18)×10- 3 mm2 /s. The cystadenoma findings were thin and smooth wall with uniocular or multicular,lacks of the mural nodule, which showed hypointensity on DWI,and the mean ADC values was (1.72±0.28)×10-3mm2 /s.In diagnosis of cystadenocarcinoma on DWI provided sensitivity 100% and specificity 93.3%,the DWI signal and the ADC value in comparison wih cystadenocarcinoma and cystadenoma have statistical differences(P<0.05). Conclusion DWI sequence play an important role in diagnosis of cystadenocarcinoma and cystadenoma,by analysis of the ADC values we found that the ADC values can provide qualitation value,and have differential value in diagnosis of cystadenocarcinoma and cystadenoma.