Objective To evaluate the application value of rescue ICSI in fertilization failure after conventional IVF and choose the best rescue window before oocyte aging according to the results of rescue ICSI performed in different time.Methods The data of 93 IVF cycles were analyzed retrospectively. Rescue ICSI was performed in these cycles after conventional IVF failure. Because of the different rescue time, these cycles were divided into two groups: early rescue group (group A,77 cycles, rescue ICSI performed 4 8 h after conventional IVF) and late rescue group (group B, 16 cycles, rescue ICSI performed 20-22 h after conventional IVF).Results There were no statistically significant differences in age of female, duration of infertility, number of oocytes retrieved every cycle. The normal fertilization rate,pregnancy rate and implantation rate were decreased in group B compared with those in group A (P<0.05). In group A, the normal fertilization rate of rescue ICSI performed 4-6 h after conventional IVF (group AI) was increased compared with that of rescue ICSI performed 6-8 h (including 6 h) after conventional IVF (group A2)(66.5% vs 55.9%)(P<0.05); while the abnormal fertilization rate in group Al was decreased compared with group A2 (9.0% vs 14.4%)(P<0.05). Clinical pregnancy rate was slight higher in group A1 than in group A2, though this failed to be significantly different.Conclusion Rescue ICSI is effective if fertilization was failure after conventional IVF,the most important thing is to choose the reasonable rescue window before oocyte aging when ICSI is performed.