The relationship between morphologic criteria including zona pellucida thickness (ZPT) and the ET outcomes was studied for day-4 fresh morula transfer (n=306). Pregnancy rate (PR) was significantly correlated with the proportion of compaction (P<0.0001), the morphology (P<0.0001) and the amount of fragmentation (P=0.0004) of the best morula. In cycles with single morula transfer (n=121), only the morphology was significantly correlated with PR (P=0.03). The average ZPT of the best morula was significantly thinner (P<0.0001) in conception cycles (7.9 ± 2.1 μm) than in non-conception cycles (9.3 ± 1.8 μm). Also in cycles with single morula transfer, the average ZPT was significantly thinner (P=0.004) in conception cycles (8.5 ± 1.9 μm) than in non-conception cycles (9.7 ± 2.0 μm). Factors influencing conception were assessed by logistic-regression analysis. The average ZPT (P<0.0001), the morphology (P=0.0003) and the amount of fragmentation (P=0.02) of the best morula were significantly related to the conception, whereas only the average ZPT (P=0.01) was related to the conception in single morula transfer cycles. We concluded that the average ZPT and the morphology of embryo seem to be strong predictors of pregnancy in day-4 morula transfer and thus important indicators of good embryo quality.