Mucinous tumors account for 10-15% of all epithelial of ovarian tumors, and 40% of them are borderline. Not many factors are known about progression into mucinous carcinoma of borderline ovarian tumors. The incidence of progression into invasive carcinoma is reported about 2.4% for borderline serous tumous, and 1.6% for borderline mucinous ovarian tumors. Mucinous tumors often exhibit a morphologic continuum of beningn, borderline, and invasive, so a pathologist should pay attention when examine the pathologic specimen not to miss carcinoma. This is the case of 54 female patients who developed invasive mucinous ovarian carcinoma 6 months after surgical treatment of borderline mucinous ovarian tumour.