Abstract： Objective To investigate the fertilization method and related factors of embryonic development competence for in vitro mature (IVM) human oocytes. Methods Between July and December, 2008, 354 immature oocytes were obtained from 135 couples during intracytoplasmic sperm injection (ICSI) procedure as required for assisted reproduction due to male infertility in our hospital. These oocytes were incubated in PI culture system and observed for their maturation at 20-24, 26-30 and 36-40 h. The matured oocytes (degenerative ones were excluded) were randomly divided into routine in vitro fertilization (IVF) group or ICSI group to receive fertilization accordingly. The rates of natural fertilization, cleavage and day-3 good embryos were compared between the two groups. Meanwhile, intra-greup comparisons were performed between patients with good embryos and non- good embryos for their age, duration of infertility, length of stimulated ovulation and time to maturation of the oocytes in vitro. Results The rate of oocytes for maturation in vitro was 82. 5% (292/354). The rate of natural fertilization was lower in IVF group than that in ICSI group [56.5% (70/124) vs 69.9% (107/153), P<0.05], and there were no significant difference in rates of cleavage and good embryos between two groups (P>0.05). Patients with good embryos in both groups were with younger age and less time to IVM than those with non-good embryos (P< 0.05). but did not differ significantly in duration of infertility and length of stimulated ovulation (P>0.05). Conclusions ICSI can improve the fertilization rate of IVM. IVM oocytes are able to be fertilized by routine IVF and the day-3 quality of embryos from routine IVF are similar to those from ICSI. Patients' age and time to maturation of the oocytes in vitro seem to be the major factors of the development competence of IVM oocytes.