Background. Determination of sex is an important concern to the forensic anthropologists as it is critical for individual identification. This study has investigated the existence of sexual dimorphism in the dimensions and the area of the IOA triangle. Methods. A total of 100 adult dry skulls, (78 males; 22 females) from departments of anatomy in Nigerian universities were used for this study. Automatic digital calliper was used for the measurement. Coefficient of variation, correlation, linear regression, percentiles, and sexual dimorphism ratio were computed from the IOA triangle measurements. The IOA triangle area was compared between sexes. Results. The male parameters were significantly (P < 0.05) higher than female parameters. The left opistocranium-asterion length was 71.09 ± 0.56 and 61.68 ± 3.35 mm and the right opistocranium-asterion length was 69.73 ± 0.49 and 60.92 ± 2.10 mm for male and female, respectively. A total area of IOA triangle of 1938.88 mm(2) and 1305.68 mm(2) for male and female, respectively, was calculated. The left IOA indices were 46.42% and 37.40% in males and females, respectively, while the right IOA indices for males and females were 47.19% and 38.87%, respectively. Conclusion. The anthropometry of inion-opistocranium-asterion IOA triangle can be a guide in gender determination of unknown individuals.