To develop novel transdermal formulation for aceclofenac, microemulsion was prepared for increasing its skin permeability. Based on solubiity and phase studies, oil and surfactant was selected and composition was determined. Microemulsion was spontaneously prepared by mixing ingredients and the physicochemical properties such was investigated. The mean diameters of microemulsion were approximately 90 nm and the system was physically stable at room temperature at least for 3 months. In addition, the in vitro and in vivo performance of microemulsion formulation was evaluated. Aceclofenac was released from microemulsion in acidic aqueous medium, and dissolved amounts of aceclofenac was approximately 30% after 240 min. Skin permeation of aceclofenac from microemulsion formulation was higher than that of cream. Following transdermal application of aceclofenac preparation to delayed onset muscle soreness, serum creatine phosphokinase and lactate dehydrogenase activity was significantly reduced by aceclofenac. Aceclofenac in microemulsion was more potent than cream in the alleviation of muscle pain. Therefore, the microemulsion formulation of aceclofenac appear to be a reasonable transdermal delivery system of the drug with enhanced skin permeability and efficacy for the treatment of muscle damage.