Transdermal drug delivery system (TDDS) is a new method for drug delivery. The analysis of plenty of experiments in vitro can lead to a suitable mathematical model for the description of the process of the drug’s penetration through the skin, together with the important parameters that are related to the characters of the drugs. After the research work of the experiments data, a suitable nonlinear regression model was selected. Using this model, the most important parameter-penetrated coefficient of 20 drugs was computed. In the result one can find, this work supports the theory that the skin can be regarded as singular membrane.