We present in this paper an analytical model of the current -voltage (I-V) characteristics for submicron GaAs MESFET transistors. This model takes account the analysis of the charge distribution in the active region and incorporate a field depended electron mobility, velocity saturation and charge build-up in the channel.we propose In this framework an algorithm of simulation based on mathematical expressions obtained previously. The results obtained of the model are discussed and compared with those of the experimental data reading obtained from the literature [1], The agreement has been shown to be good.