Objective To analyse factors influencing the outcome of frozen-thawed embryo trans- fer (FET) Method A retrospective analysis was performed in our center on 129 thawing cycles from March 2001 to April 2003. The related parameters were compared between conceived and non-conceived cycles. Results There were totally 129 clinical pregnancies in these transfers (pregnancy rate: 27.1%). Frozen-thawed embryos were transferred to natural cycles and CC cycling and hormone replacement treatment had equal success. Groups of IVF and ICSI did not differ significantly in pregnancy rates (P>0.05). The pregnancy rates for one, two, three and four pre-embryos transfer were 0, 20.0%,44.1% and 75.0%, respectively (P<0.05). There were statistical differences between pregnancy group or non- pregnancy group in the endometrial thickness, CES, CES/No. of embryo. A higher pregnancy rate was observed in embryo transfers which had at least one 4-cell grade I embryo (d 2)(P<0.01). Conclusions The most important factors influencing the implantation rate and preg- nancy rate of frozen-thawed embryo transfer are age, endometrium thickness,and the number, morphology and growth rate of transferred frozen embryos of women participants. Objective To analyze the factors influencing the outcome of frozen-thawed embryo trans-fer (FET) Method A retrospective analysis was performed in our center on 129 thawing cycles from March 2001 to April 2003. The related parameters were compared between conceived and non-conceived cycles . Results There were totally 129 clinical pregnancies in these transfers (pregnancy rate: 27.1%). Frozen-thawed embryos were transferred to natural cycles and CC cycling and hormone replacement treatment had equal success. Groups of IVF and ICSI did not differ significantly in pregnancy The pregnancy rates for one, two, three and four pre-embryos transfer were 0, 20.0%, 44.1% and 75.0%, respectively (P <0.05). There were statistical differences between the pregnancy group or non - pregnancy group in the endometrial thickness, CES, CES / No. of embryo. A higher pregnancy rate was observed in embryo transfers which had at least one 4-cell grade I embryo (d 2) (P <0.01) important f actors influencing the implantation rate and preg- nancy rate of frozen-thawed embryo transfer are age, endometrium thickness, and the number, morphology and growth rate of transferred embryos of women participants.