Purpose To investigate the clinicopathological features,immunophenotype,molecular changes,differential di-agnosis,treatment and prognosis of the micropapillary subtype of serous borderline tumor(MSBT)in the ovary.Methods The clinical and pathological data of 14 cases of ovarian MSBT.Im-munohistochemical EnVision staining was used to detect the ex-pression of IMP3.BRAF and KRAS mutations were detected by qRT-PCR and Sanger sequencing,respectively.Its clinical and pathological characteristics were analyzed with review of relevant literature.Results The age of the patients ranged from 27 to 56 years,with mean 41.7 years.Nine cases had bilateral ovari-an masses.Preoperative serum CA125 increased in 11 cases.On gross examination,the cut section was cystic and solid with intracystal papillae.Microscopically,all cases showed a papilla-ry structure,with the characteristic elongated micropapillae radi-ating directly from the cyst wall or large unbranched papillae.The length to width ratio of the papillae was greater than 5.The cells covering the papillae were cubic to polygonal.Mild to mod-erate atypia was noted with a range of＞5 mm in the micropapil-lary area.Five cases had microinvasion.Six cases had non-in-vasive peritoneal seeding.Five cases were accompanied by asci-tes,and atypical tumor cells were observed in ascites.Three ca-ses had lymph node involvement.Nine cases had psammoma bodies.Immunohistochemically,the tumor cells were positive for ER,PR,CA125,CK7 and WT-1;p53 was wild type,HER2 and IMP3 were negative,and Ki67 was positive in 5％to 30％.KRAS mutations were detected in 3 of 14 cases,inclu-ding G12C,G12D and Q70(nonsense mutation).No BRAF V600E mutation was detected,and 1 case had BRAF T559I mu-tation.Seven patients underwent radical surgery and 7 patients underwent conservative surgery without special treatment after surgery.Five patients had a history of recurrence,and the fol-low-up time ranged from 1 to 12 years.Conclusion MSBT has special morphology,often bilateral,and is prone to peritoneal implantation and recurrence.It should be distinguished from classical ovarian serous borderline tumor.