A fuel property detecting system comprises a control unit coupled to a fuel injector and an air-fuel ratio sensor. The control unit is arranged to sample data of a response wave-form of an exhaust air-fuel ratio in response to an injected fuel quantity at a transient period, to identify a plant model as to fuel in use by controlling a parameter of a previously constructed plant model on the basis of the input and output data so as to decrease a prediction error between the plant model and a norm model, to calculate a cutoff frequency of the identified plant model, and to estimate a fuel property of the fuel in use from the calculated cutoff frequency of the identified plant model and data previous stored in the control unit.