The purpose of the present study was to design an optimal ketoprofen gel with appropriate penetration rate, shortened lag time and acceptable skin irritation. The combination of different mechanism enhancers including nonivamide, menthol and ethanol were used as multi-enhancers for producing a synergistic enhancement effect and reducing the skin irritation via diminishing the used amount of enhancers. The central composite design was applied to prepare a systemic formulation. The penetration rate (PR), lag time (LT) and skin irritation score (TIS) of a commercial product (Formax plus gel containing 3% ketoprofen) were determined by in vivo study and used as a criterion for designed formulations. The PR, LT and TIS of commercial product were 462.2+/-162.5 microg/h, 0.6+/-0.1 h and 12.7+/-0.6, respectively. Among these designed experimental formulations, four formulations including F07 (code: -1/+1/-1), F11 (code: +1/+1/-1), F13 (code: 0/0/-1.732) and F14 (code: 0/+1.732/0), their PR was not smaller and LT and TIS were not greater than that of commercial product, indicating that these experimental ketoprofen gels could be used in the clinical situation.