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Background and Aims: Little information is available for investigating the effect of the direction of sperm injected into the cytoplasm on clinical results for human oocytes following intracytoplasmic sperm injection (ICSI). This study aimed to examine the impact of the direction of sperm (head-first or tail-first) injected into the cytoplasm on fertilization and embryo development for high-elasticity or low-elasticity human oocytes following the Piezo-ICSI. Methods: We retrospectively investigated 2717 high-elasticity oocytes and 907 low-elasticity oocytes between May 2016 and December 2022. Of 2717 high-elasticity oocytes, 1148 were injected sperm head-first, and 1569 were injected sperm tail-first into the cytoplasm by Piezo-ICSI. Of 907 low-elasticity oocytes, 480 were injected head-first, and 427 were injected sperm tail-first into the cytoplasm. The membrane of high-elasticity oocytes was robust and was ruptured artificially by the piezo-pulse. The membrane of low-elasticity oocytes was weak and spontaneously ruptured without the piezo-pulse. The rates of degeneration, fertilization (2PN), 0PN, 1PN, [Formula: see text]3PN, abnormal cleavage, and good-quality day-3 embryos were evaluated in both groups. Results: Table 1 shows the effect of sperm direction on fertilization and embryo development for high-elasticity oocytes. The fertilization rate of tail-first group was significantly higher than the head-first group. The 0PN rate of tail-first group was significantly lower than the head-first group. The good quality Day-3 embryo rate of tail-first group was significantly higher than the head-first group. Table 2 shows the effect of sperm direction on fertilization and embryo development for low-elasticity oocytes. The fertilization rate of the tail-first group was significantly higher than the head-first group. However, no significant difference was observed in other comparable items. Conclusions: Tail-first sperm injection significantly improved the fertilization rates more than the head-first sperm injection regardless of the elasticity of the oocyte membrane following Piezo-ICSI.