The motor current signature analysis (MCSA) method for diagnosing broken rotor bars faults in squirrelcage motor had some drawbacks that the diagnostic effects were subject to running state of the motor, which could even lead to a wrong diagnosis of the fault. A novelty approach based on the instantaneous frequency analysis of the startup transient was presented. The proposed approach consists of extracting the monocomponent signals from the startup current by FIR lowpass filtering technique and calculating the instantaneous frequency of the obtained signal, via the derivative of the phase of the analytical signal, and according to the characteristic pattern of the IFson the fsplane, it was judged if the broken rotor bar defect occurs. The experiment verification was performed on a 3 kW motor test bench; the experiment results proved the effectiveness of the proposed method.