Microemulsion formulations, which can be used to improve the bioavailability of poorly soluble drugs, were designed using only pharmaceutical excipients. Several types of oils and surfactants were tested and it was found that propyleneglycol monoalkyl ester and glycerol monoalkyl ester were solubilized easily in an aqueous medium by various types of surfactants. Although propyleneglycol dialkyl ester was difficult to be solubilized, the solubility was significantly enhanced by mixing it with glycerol monoalkyl ester at the ratio of 1:1. The most suitable surfactants for preparing microemulsion formulations were HCO-40, HCO-60, Tween 80, BL-9EX and Pluronic P84. The use of additional surfactants such as sodium dodecyl sulfate or sodium deoxycholate significantly improved the solubilization capacity of the oils, although formulations free of these surfactants were also available. These microemulsion formulations can be administered as a form of water-in-oil microemulsion or surfactant-oil mixture, and are expected to convert to oil-in-water microemulsion in the small intestine.