Abstract: Supply voltage flicker might adversely influence the rotor fault online detection of induction motors, making the motor current signature analysis (MCSA) based detection method ineffective. Through theoretical analysis, this paper reveals the mechanism that supply voltage flicker exacerbates the performance of rotor fault online detection. We present an online rotor fault detection method that is immune to supply voltage flicker. This method comprises two main steps: first, a preliminary judgment of the rotor's health or fault status is made based on the primary characteristic components of rotor faults; second, the preliminary judgment is further verified using auxiliary components that are uniquely associated with rotor faults. Simulation and experimental results demonstrate the effectiveness of the presented method.