Microemulsion systems, composed of water, isopropyl myristate (IPM), polyoxyethylene sorbitan trioleate (Tween 85 ), and ethanol, were investigated as transdermal drug delivery vehicles for a lipophilic model drug( meloxicam). The purpose of this study was to investigate the physicochemical properties of the tested microemulsion and to find the correlation between the physicochemical properties and the skin permeation rate of the microemulsion. Pseudo-ternary phase diagram of the investigated system at a constant surfactant/cosurfactant mass ratio (Km =1:1) was constructed by titration at 20℃, and the five formulations were selected for further research in the o/w microemulsion domains. The values of electrical conductivity and viscosity showed that the selected systems were bicontinuous or non-spherical o/w microemulsion, and the electrical conductivity and viscosity were increased with increasing the content of water. These results suggest that the optimum formulation of microemulsion, containing 0. 375 meloxicam, 5% isopropyl myristate, 25% Tween 85, 25% ethanol, and water, showed the maximum permeation rate. It had a high electrical conductivity, small droplet size, and proper viscocity.