Concept of the fertilizing ability of human spermatozoa has been drastically changed since intracytoplasmic sperm injection (ICSI) played an important role of modern reproductive medicine. Recent studies on the qualities of sperm nucleus revealed that the fertilization and embryo development in vitro should be related to the structure of sperm nuclear chromatin, especially in case of ICSI procedure. It has been suggested that oocytes injected with sperm nuclei with poor S-S bonds have more developmental potency than that with S-S rich sperm nuclei. Moreover, it has been pointed out from the assessment of DNA fragmentation that oocytes injected with genetically impaired nuclei have little potency to develop beyond 8 cell stage embryo. Hereafter, the more definitive assessments of sperm nuclear chromatin should be needed to select the best nuclei for ICSI procedure.