ABSTRACT Background: Transfer of embryos at the blastocyst stage has been associated with exceptionally high implantation rates. There are, however, only a prospective randomized studies comparing day 3 versus day 5 embryo transfers. Furthermore, the number of embryos replaced in the day 3 group transfer is often higher than the number of blastocysts replaced, thereby affecting implantation rates-Subjects & Methods: A total of 86 patients undergoing standard Intracytoplasmic sperm injection who had developed at least five 8-cell embryos, symmetrical cells and showing <11% extracellular fragmentation on day 3 were randomized for day 3 or day 5 transfer. In this prospective, randomized study the implantation and pregnancy potential of embryos transferred on day 3 or day 5 were compared-Results: There was statistically significant difference between day 3 and day 5 transfer regarding clinical pregnancy rates (29.5% versus 66.7%), implantation rates (7.9% versus 34.7%), twinning rates (2.3% versus 11.9%)-Conclusion: Higher significant difference in clinical pregnancies,implantation rate in blastocyst transfer group than cleavage transfer group so it is possible to decide on performing blastocyst transfer when having 5 or more fertilized oocytes with 8-cell embryos, symmetrical cells and showing <11% extracellularfragmentation on day 3, without risking for a transfer cancellation.