An accurate determination of the series resistances with physical meaning is extremely important for the characterization and modeling of FET's. In this paper, we discussed the bias-dependent behavior of the source and gate resistance of GaAs MESFETs. A new analytical method is presented to extract these resistance at all bias conditions from measured S-parameters. This method enables the calculation of the equivalent circuit elements quickly. An application of this new technique to extract the source and gate resistance of GaAs MESFET reveals a bias-dependent behavior for both resistance. The influence of bias on these resistance is significant under all bias conditions and must be carefully estimated if an accurate modelling of the MESFET behavior is required.