Ecdysteroids and brassinosteroids are steroid hormones that regulate insect molting and plant growth, respectively. In this study, novel ecdysteroids and brassinosteroids were designed and synthesized, and their biological activity was evaluated. Structure-activity relationship details between the structure of the steroid skeleton of ecdysteroids and their molting hormone receptor binding activity, as well as between the side chain structure of brassinosteroids and their plant hormonal activity were investigated. The results of these investigations suggested that in order to exhibit hormonal activity in insects or plants, the structure of the side chain in these hormones should be more important than that of the steroidal core skeleton.