A microemulsion vehicle had been studied as a possible matrix for transdermal delivery of theophylline. The existence of microemulsion regions were investigated in pseudo-ternary phase diagrams, and various microemulsion formulations were prepared using oleic acid, Cremophor RH40/Labrasol (1:2) and water. The optimal formulation of the microemulsion was evaluated in vitro using Franz diffusion cells. The droplet size of microemulsion was characterized by photo correlation spectroscopy. Pharmacokinetic study in vivo was conducted using rabbits, and the results indicated that AUC(0-->infinity) of transdermal administration was 1.65-fold higher than that of oral solution administration. These studies showed that microemulsion system of theophylline might be promising vehicles for the transdermal delivery of theophylline.