We observed the crater defects (CD) of human sperm head at a magnification of×1,000 with an oil immersion lens and using an inverted microscope equipped with Nomarski optics and scanning and transmission electron microscope. Motile and normally shaped sperm with three different sizes of CD were selected and injected into M-II oocytes, donated by a consenting human patient and superovulated mice. On average, 1.54 CDs were confirmed in almost all (95.9%) sperm head, and the incidences of CDs did not change after the various demembraned pretreatments. The CDs were also found in epididymal and testicular sperm and elongating and elongated spermatids, but the incidences were a little fewer than ejaculated sperm. No CDs were found in mouse and boar sperm irrespective of the pretreatments. The fertilization and blastocyst formation rates were not significantly different following the ICSI using human sperm with 3 sizes of CDs. We speculate that CDs are not abnormal findings and physically structural changes during sperm maturation process.