It has been reported that the rate of blastocyst formation after ICSI is lower than that after conventional in vitro fertilization IVF. The aim of this study is to assess relationship between maturity of chromatin structure of spermatozoa and embryo development. Samples; Ejaculated sperm was collected from 21 patients treated with ICSI and 22 patient treated with IVF from January to October in our hospital. N-ethylmaleimide (NEM) was used to prevent natural oxydation. Centrifugated and washed sperm was mixed with 5 mM NEM and stored at −80°C until analysis. Two hundred μl of thawed sperm aliquot was suspended in same dose of low pH detergent for 30 sec followed by suspension with 1.2 ml of acridine orange (AO) working solution for 3 min. FACScan equipped with CELL QUEST (BECTON DICKINSON) was used for analysis. Cells outside the main population (COMP; %) was measured for SCSA parameter. Although there are no corelation in conventional IVF (r=0.03, p=0.899, n=21), COMP in ICSI has positive corelation with the rate of blastocyst formation (r=0.477, p=0.025, n=22). In ICSI, the number of disulfide bond in sperm chromatin might effect on dyssynchrony during process of pronuclear formation and embryo development.