Aceclofenac is an non-steroidal antiinflammatory drug which has been used in the treatment of rheumatoidal rthritis and osteo-arthritis. In order to decrease the gastric ulcerogenic effects and contol the plasma level of aceclofenac, we have developed the transdermal delivery system of aceclofenac plaster, which were formulated employing matrix polymers of acrylates and penetration-enhancers such as $Lauroglycol^{circledR}$, $Transcutol^{circledR}$, oleic acid and linoleic acid. Using Franz diffusion cells mounted with a rat skin, transdermal penetration characteristics of the formulations were evaluated by the HPLC assay of aceclofenac and diclofenac, an active metabolite, in the receptor compartment of pH 7.2 phosphate buffered solution. Skin penetration was increased when the content of aceclofenac increased, showing the flux $(J,;{mu}g/cm^2/hr)$ of 0.37 and 2.50 for 2% and 6.75% of the content, respectively. The flux$(J,;{mu}g/cm^2/hr)$ from plasters made of $Durotak^{circledR}$ 87-2074, $Durotak^{circledR}$ 87-2510 and $Durotak^{circledR}$ 87-2097 were 2.50, 2.77 and 4.39, respectively. $Durotak^{circledR}$ 87-2074 showed the lowest penetration due to the carboxylic acid group in the polymer, which might form a strong hydrogen bonding with a secondary amine of aceclofenac. Although both $Durotak^{circledR}$ 87-2510 and $Durotak^{circledR}$ 87-2097 are amine-resistant adhesives, $Durotak^{circledR}$ 872510 showed lower penetration than $Durotak^{circledR}$ 87-2097 because of the hydroxyl group in $Durotak^{circledR}$ 87-2510, which might form a weak hydrogen bonding with aceclofenac. These results reveal that the functional group in acrylic polymers would greatly affect the release of aceclofenac from the matrix, which is the rate-limiting step in the penetration of aceclofenac through rat skins. The penetration of aceclofenac from plasters using different penetration-enhancers increased in the following order: Transcutol < linoleic acid < oleic acid. And the flux from the plasters containing oleic acid as a penetrationenhancer was 2.22 times greater than that of creams, which suggest that a newly deveolped aceclofenac plaster could be used in the treatment of rheumatoidal arthritis and osteo-arthritis as an advanced transdermal delivery system.