A method of determining a failure of an active air flap, including determining whether or not an active air flap is in a non-openable state, if the active air flap is in the non-openable state, continuously checking a variation of the temperature of engine-cooling water, and if the variation is below a reference variation, interrupting the generation of failure-alert, and if the variation of the temperature of engine-cooling water is above the reference variation, processing whether to generate the failure-alert such that if the time taken to reach the variation is above a reference temperature time, interrupting the generation of failure-alert, and if the time is below the reference temperature time, generating the failure-alert.