It has been reported that high quality embryos can be selected using the Graduated Embryo Score (GES) on day2 or day3. However it is still unknown whether GES is useful for the selection of day5–6 blastocyst stage embryos. Blastocyst stage embryos were evaluated with the conventional Gardner's classified system on day5 following evaluation with the modified GES system on day2–3, and were then transferred. The pregnancy rate (47.4%) was significantly higher for transferred blastocysts with high GES points (GES ≥ 70) than the pregnancy rate (16.7%) of transferred blastocysts with low GES points (GES < 70) which had been evaluated as high potential quality blastocysts by Gardner's classification. These results suggest that the GES system is useful for the selection of high potential blastocysts in blastocyst transfer cycles. Moreover, the GES may help to promote elective single blastocyst transfer.