Abstract: To research the effect of transfer characteristic of graphene transistors by different test methods, change rules of Dirac points are researched, under the condition of that the scan range of gate voltage is between 0 V and 5 V. The changes present the following regularity. The Dirac point voltages increase gradually with the increase of scan times, when forward or backward sweeps are performed continuously. Before measurement, gate voltage hold is performed, then the Dirac point voltages have no changes when gate voltage sweeps with hold by 0V, but they increase with hold by 5V. When measurements are mutual independent, the Dirac point voltages with forward sweep are less than that with backward sweep. The first measurement can affect the second measurement when the double tests are performed. From the phenomenon above, Dirac point voltages are related to gate voltages that have swept. These properties are attributed to that electronics inject traps in surface of HfO₂. These finding and analysis are significant in the graphene reliability, the control of graphene conduction characteristics, and the realization of graphene circuits.