In order to explore the mechanism of decreased uterine bleeding of women with FICu-IUD, 48 women were divided into three groups and inserted FICu-IUD, FCu-IUD or TCu220C respectively. The ultrastructure of endometrium of the women was observed and the contents of PG_s (6-keto-PGF_(1a), TXB_2, PGE_2, PGF_2),the activities of t-PA and PAI, and the content of vWF in the uterine flushing of the women were determined before and after Inserting IUDs. The results were shown as follows: The endometrial impairment caused by FICu-IUD was mild as compared with that by FCu-IUD and TCu220C. After FCuIUD and TCu220C insertion for 1, 3, 6 months, the 6-keto--PGF_(1a)/TXB_2 ratio was increased very significantly (P<0. 01 or P<0. 05), the PGE_2/PGF_(2a) ratio has not changed distinctly, the activity of t-PA ascended remarkably (P<0. 01), but the activity of PAI and the content of vWF didn't change significantly. Before and after insertion of FICu-IUD, the various indexes in the uterine fluid of the women had no obvious changes. Comparing with those of the insertion of FCu-IUD and TCu220C at the same period, 6- Keto-PGF_(1a)/TXB_2 ratio and the activity of t-PA was increased with the difference being significant. It was concluded that indomethacin released by FICu-IUD in the uterine cavity could lighten the endometrial damage and foreign body reaction caused by Cu-IUD, decrease the rising of the PGI_2 level and the t-PA activity and increase the TXA_2 level, which is considered to be the partial mechanism of decreased uterine bleeding by FICu-IUD.