Integration of electric vehicles (EVs) are exponentially increasing in the global market and by enabling vehicle-to-grid (V2G) EVs can inject power back into the grid. However, in an event of unintentional islanding, injecting power into the grid may causes potential safety threats to people, equipment, and power system. This paper proposes an adaptive reactive power mismatch method to detect islanding events. When islanding occurs, the proposed method drifts the system frequency away from the nominal value. Then the islanding event is detected based on frequency variations. Results show that the proposed method effectively detects islanding event within 0.801 milliseconds and have negligible non-detection zone.