Flexible Gonadotropin-releasing hormone (GnRH) antagonist protocol has been used in controlled ovarian hyper-stimulation (COH) for several years, but it is still unknown when the appropriate timing for antagonist administration is. The objective of this study was to investigate the association between the leading follicle diameter and IVF/ICSI outcomes. This retrospective study was performed in a reproductive Medicine Center of a university hospital. Four hundred and eighty infertile women who underwent IVF/ICSI using flexible GnRH antagonist protocols were divided into 3 groups according to their leading follicle size (<12 mm, 12-14 mm and >= 14 mm) on antagonist initiation day. Clinical pregnancy rate was used as the primary endpoint. The clinical pregnancy rate of <12 mm group (11.3%) was significant lower than 12-14 mm group (29.7%) and >= 14 mm group (41.8%). The implantation rate of <12 mm group was lower but the E-2 concentration on the HCG day, the number of oocytes retrieved and the number of cleavage embryos of <12 mm group were significantly higher than those in 12-14 mm group and >= 14 mm group. The <12 mm group had significant more 2PN embryos than the >= 14 mm group. The miscarriage rate and ectopic pregnancy rate among three groups were similar. Regression model showed that age was a negative factor whereas leading follicle size was a positive factor on IVF/ICSI outcome. In conclusion, our data indicates that the diameter of leading follicle >12 mm on the antagonist initiation day may lead to higher pregnancy rate.