Introduction: The types and incidence of the chromosomal abnormalities in failed fertilized oocytes that were taken after routine in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI) programs were investingated in patients referred the Royan Institute. Materials and Method: In this study, 364 air-dried preparation of failed fertilized oocytes after either IVF (91 samples) or ICSI (273 samples) were analysed. The zona pellucida of the oocytes were removed by thyrod's acid. The oocytes were subjected to a hypotonic solution followed by sequential fixation in fixatives consisted of methanol, acetic acid and distilled water. After staining in 10% Giemsa, oocytes were analysed under a light microscope at/Far/* /Lat/1000 magnification. Results: The result of the analysed oocytes, 39% were haploid and 61% aneuploid. Other types of chromosomal abnormalities such as structural aberrations (5.2%), polyploidy (2.81%), sperm chromosome premature chromosome condensation (PCC) (14.3%), oocyte chromosome stikiness (5.8%) and decondensation of oocyte chromatin (6.6%) were also observed. There was no statistical difference between the incidence of chromosomal abnormalities in failed fertilized oocytes following IVF or ICSI procedures. The frequency of sperm chromosome PCC was higher in ICSI then IVF procedure (P<0.01). Conclusion: The results indicate that chromosomal abnormalities are the major cause of oocytes unfertilization. The types and ferquency of chromosomal aberrations depends on several factors such as; nondisjunction during anaphase I and II, which cause aneuploidy; physical and chemical agents; hormone therapy, and genetic background of the patients. Hence, the frequency of chromosomal abnormality is different for different population. Results also show that occurence of aneuploidy is the main cause of failed fetilization after IVF and ICSI procedures.