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Device instabilities of graphene metal-oxide-semiconductor field effect transistors such as hysteresis and Dirac point shifts have been attributed to charge trapping in the underlying substrate, especially in SiO2. In this letter, trapping time constants around 87 μs and 1.76 ms were identified using a short pulse current-voltage method. The values of two trapping time constants with reversible trapping behavior indicate that the hysteretic behaviors of graphene field effect transistors are due to neither charge trapping in the bulk SiO2 or tunneling into other interfacial materials. Also, it is concluded that the dc measurement method significantly underestimated the performance of graphene devices.