Abstract: To study the technique of fluorescence in situ hybridization (FISH) and its application value in the diagnosis of sex chromosomal count abnormality in ovarian carcinoma cell. Biotin labeled alpha satellite X chromosome DNA(pBamX7) probe was hybridized with pre-treated slides of ovarian carcinoma cell interphase nucleus in 18 cases of ovarian carcinoma specimens. The slides were treated with Avidin-FITC and Anti-avidin, amplified with an additional layer and counter-stained with PI in antifade solution. The hybridization signals as well as interphase nucleus settings were observed with WIB filters under fluorescence microscope Olympus AX-70, and the number of interphase nucleus in the ovarian carcinoma cell was counted. It was observed under the microscope that the Biotin labeled pBamX7 probe showed green hybridization signals. Cytoplasm counter-stained with PI showed reddish orange. Increased chrosome X copy number was observed in 11/18(61%) ovarian carcinoma specimens, of which the rest 7 (39%) had no increase of chrosome X copy number. Gain of X chrosome had a certain incidence in ovarian cancers, which played a role in the recurrence and development of ovarian cancers. Its significance needs further investigation.