It has been known that estradial(E2) level is concerned with the quality, number and growing of follicles. Determination of the serum levels of E2 has been used to predict the oocyte maturity. We experienced the rare case who has significantly low serum E2 level(551.26 pg/ml) in spite of multiple ovarian responses by sonographic finding, but from her we obtained 23 oocytes on MCD#16 after COH with ultrashort protocol(HP-FSH 150 IU, daily). The numbers of retrieved oocyte were 23 and fertilization rate was 56.6%, so the result of IVF-ET was successful twin pregnancy. Serum estradiol level and sonographic finding are essential to measure the number and growing of follicles, but E2 levels may be too unreliable to be used as sole parameter for the number and maturity of oocytes.