Microemulsion (ME) is a micron-sized droplet that consists of oil and water, with addition of a surfactant and cosurfactant. Recently, ME is widely used in biomedical application for proper drug delivery in the human body. Castor oil as the oil phase, Tween 80 or Tween 20 as the surfactant, glycerol or ethanol as the cosurfactant, and DI water as the water phase were used for ME preparation in this study. The effect of the surfactant-to-cosurfactant ratio on the pseudoternary phase diagram was investigated. The as-synthesized ME with the composition of 5 wt.% castor oil, 85 wt.% surfactant mixture, and 10 wt.% water was characterized based on its particle size, polydispersity index, and zeta potential. From that composition, the largest ME was attained at an S-mix 2 weight ratio of tween 80 to ethanol. Astaxanthin as lipophilic drug substance was used as the model drug for the ME encapsulation study. The thermal and storage analysis test of ME and astaxanthin-loaded ME demonstrated the stability of the as-synthesized ME and its analogous drug-loaded form.