Several researches have been done on the establishment of normal aortic diameters at various levels. These researches have been done on the Caucasians as few studies have been within Africa using Africans. This work was designed to establish the transverse aortic arch diameters of Nigerians within the southeast and to establish the relationship of these values with the heart diameter, age and sex. PA chest radiographs of 1018 individuals of age 4 - 80 years were used for this study. The research presented a mean value of 4.7+0.46 for the aortic arch diameter of the Nigerian population. Mean aortic arch diameters for various age groups have been established. The research also set lower and upper limits of normal aortic arch diameter for the two sex groups, the entire distribution and for the distribution of males and females above age 20. The research noted higher values of aortic arch diameter in males than in females of same age group. Age and heart diameter showed strong, positive statistically significant relationships with aortic diameter. This diameter was noted to be slightly less than a third of the heart diameter within all the age groups.