To understand the auto-ignition behavior in response to the flow turbulence, the effects of scalar dissipation rate fluctuation on the ignition of a nonpremixed hydrogen/air mixture is studied using detailed chemistry in a counterflow configuration. Unsteady scalar dissipation rate is imposed in a sinusoidal form by oscillating the velocity at the nozzle inlet. Mean scalar dissipation rate is chosen such that it is very close to the steady ignition limit, and instantaneous scalar dissipation rate becomes higher than the steady ignition limit for some duration during the induction period. Ignition delay response to frequency of the imposed scalar dissipation rate oscillation is studied for two distinct cases, depending on whether the time average of a cycle of scalar dissipation rate oscillation at ignition kernel is (a) less or (b) greater than the steady ignition limit. For low frequencies, the ignition delay response for both cases is quasi-steady in that it correlates well with the mean scalar dissipation rate. However, at high frequencies the ignition delay response is significantly different for the two cases. For case (a), the ignition delay increases with frequency and levels off at higher frequencies. On the other hand, for case (b) ignition delay increases monotonically with frequency up to a critical value, beyond which no ignition is observed. The high frequency behavior is attributed to the excursion time effect. A newly defined ignition parameter is proposed based on the ignition kernel Damk̈ohler number such that all the unsteady effects of scalar dissipation rate oscillation on ignition delay can be uniquely mapped to this parameter. Subsequently, a new criterion for ignitibility is proposed based on this parameter.