The secant law is the relationship between the frequencies of a vertically incident wave and an obliquely incident wave that are reflected from the same level (i.e., density) of a stratified ionosphere. This thesis investigates the validity of the secant law applied to sporadic E. Sporadic E data from two backscatter sounders and one vertical incidence ionosonde located in Japan were studied to test the validity of the secant law. The B-scan photos of the backscatter sounders were searched for Es patches which were then compared with the sporadic E parameters defined for vertical incidence ionograms. Finally, two theoretical density models of Es were analyzed to predict the type of returns expected from the signals of backscatter sounder and vertical incidence ionosonde. These models included the partial reflection and scattering mechanisms that might be appropriate for sporadic E.