At present,the method of obtaining the field-effect mobility of organic thin film transistors (OTFT) is mainly to calculate the field-effect mobility according to the electrical characteristic curves of OTFT.The method is studied deeply in this paper.It is found that the f ield-effect mobility of OTFT is related to its operating state.In the linear operating state,the field effect mobility of increases with the increa se of gate voltage. Therefore,when the linear field effect mobility is used to represent the electr ical properties of an OTFT,the corresponding gate voltage must be indic ated.In the saturated operating state,when the leakage voltage is greater then 1.5times of the movximum gate voltage,the field effect mobility tends to be a certain value.Using this value to characterize the electrical properties of OTFT is more objective and accurate.